UNSC Marine Corps

The UNSC Marine Corps is a branch of the United Nations Space Command Defense Force, mainly a branch of the UNSC Navy, primarily responsible for land-based military operations, and protecting Navy star ships and bases from attack.

Background
A rugged and diverse assortment of well equipped and highly trained men and women, the Marine Corps of the United Nations Space Command Defense Force are fighting a losing war against the Covenant's superior technology and numbers. Under the leadership of notable commanders, they continue to wage a furious struggle against the Covenant, even as their numbers dwindle. texas sucks shit

Before The Human-Covenant War
The UN Marine Corps (the predecessors of the UNSC Marine Corps) was one of the main combat groups in the Interplanetary War. In 2163, Marines were first deployed outside of Earth on Mars. After the UN won the war in 2164, they became the UNSC, with the Marines becoming the UNSC Marine Corps.

First Encounters
The first documented Marine Corps encounter with the covenant was the Battle of Harvest, where the UNSC Marine Corps deployed to battle Covenant ground forces on the colony after it was glassed by the Covenant Fleet.

Another one of their first encounters occurred on an arctic world, possibly in the Harvest System, where the Marine Corps quickly established a home base while an outfit of Marines reconnoitered the nearby area, but were slaughtered by camouflaged Energy Sword-wielding Special Operations Elites. In response, a detachment of Scorpions and Warthogs were sent to investigate, but were too late, and were met, head on, only by a heavy Covenant battle force, with large numbers of Banshee attack fighters and Elite warriors, and the UNSC suffered heavy air casualties. Later, the Marine presence was reinforced by Spartan Group Omega, but the outcome of this battle was unknown.

Battle of Sigma Octanus IV
There was a Marine presence on Sigma Octanus IV. In the first stage of the battle, during a naval engagement, a Covenant Carrier was able to land a large contingent of troops on the northern pole of the planet.

The Covenant were able to take Côte d'Azur, a major population center. Fortunately, almost all of the civilian population was able to be evacuated beforehand, and UNSC Marine Corps General Kits launched an assault by 0700 hours to retake the city. It was unsuccessful.

Meanwhile, three Marine squads, nearby Côte d'Azur and under the command of Lieutenant McCasky, were dispatched for a light reconnaissance of Grid 13 x 24. They were engaged by Covenant forces, including Hunters, and a sole UNSC M12 Warthog LRV of the reconnaissance force survived with four Marines by the names of Harland, Walker, Fincher, and Cochran. Shortly afterwards, Firebase Bravo was destroyed by the Covenant, and the Warthog was forced to divert to Alpha Headquarters.

It, too, had been attacked, though the Covenant attack had been repulsed, though not without suffering heavy casualties. Harland promptly requested reinforcements from the cruiser in orbit, the. The Leviathan sent a team of twelve SPARTAN-IIs commanded by SPARTAN-117 to reinforce.

The surviving Marines of Marine HQ protected a Pelican Dropship while the SPARTANs detonated a HAVOK Nuclear Warhead in Côte d'Azur, cleaning the Covenant ground presence from Sigma Octanus IV, and then retreated back to a Pelican dropship, which promptly took off with other Pelicans holding other Marine survivors for the UNSC Leviathan.

Battle of Reach
The UNSC Marine Corps maintained a sizable presence of Reach when the Covenant stormed the planet, bypassing the defense fleet in orbit and landing large numbers of soldiers on the north and south poles.

By 0555 hours, the Covenant had overran FLEETCOM HQ on Reach, destroying the perimeter and swarming the Marine defenders with Banshees, Wraiths, and battalions of infantry. The objective of the Covenant ground armies was to destroy a series of fusion reactors on Reach's surface that powered the orbiting Super Magnetic Accelerator Cannon-equipped Orbital Defense Platforms, and thus, disabling the UNSC's sole major defensive asset and allowing the Covenant fleet to occupy Reach's airspace.

By 0622 hours, the situation was critical enough that the sent a full detachment of SPARTAN-II supersoldiers to Reach in a Pelican drop ship to defend the fusion generators. The Orbital Defense Platforms were already being vaporized in space, and if they were compromised from the ground, the meager remaining UNSC defense fleet would swiftly fall.

Meanwhile, Charlie Company, a Marine detachment, was in charge of defending Orbital Defense Generator Facility A-331. They were forced to defend against a massive Covenant attack, using chain guns and missiles to fight off superior forces. Lieutenant Buckman, the CO of Charlie Company, flanked the Covenant, but FLEETCOM, alarmed that the Covenant might overrun ODG A-331, overreacted and carpet-bombed the area, eliminating the Covenant as well as most of Charlie Company.

There were four survivors of Charlie Company that held their position until they met with the SPARTANs. The survivors were sent with several heavily wounded SPARTANs to a fall back position at Mechanite Mountain, but all four Marines died from the Covenant, ending the UNSC Marine Corps's known involvement in the Battle of Reach.

Landfall
A sizable Orbital Drop Shock Trooper special forces detachment and a standard Marine detachment was assigned to the for its intended operation to infiltrate Covenant High Command and kidnap a Prophet as leverage for negotiating a peace treaty between the UNSC and the Covenant. However, the ambush over Threshold forced the Pillar of Autumn to crash land on the Forerunner installation also known as Installation 04 (Alpha Halo). The Pillar of Autumn's ODST detachment consisted of four 130-man companies: Companies A, B, C, and D, for a total of approximately 520 operators. During the Covenant boarding of the Pillar of Autumn, they were assigned to defend the engine room of the warship against the boarding parties. They were extremely efficient in their defense, suffering only several dozen casualties while successfully holding off the Covenant at the two entrances to the massive engine chamber. However, the standard Marines did not fare as well in their defense, setting up multiple lines of defense around the periphery of the Pillar of Autumn and utilizing Combat Barriers and urban combat techniques in the restrictive environment to hold off the enemy, which were Elites and Grunts. Ultimately, they were unsuccessful, and the Covenant managed to infiltrate many compartments of the warship before Captain Jacob Keyes, commander of the Pillar of Autumn, decided it was a lost cause to hold off superior numbers of attacking Covenant warships as well as internal boarders and ordered all hands to evacuate. .

Naval crewmen and standard Marines evacuated in Bumblebee life pods while the ODST companies utilized Human Entry Vehicles. The landings, however, had numerous problems and killed dozens of UNSC soldiers and crewmen alike. Major Antonio Silva, the ODST Commander, ordered all surviving ODST units to secure a lightly-defended Covenant-held butte, which he turned into a coordinating home base for the UNSC resistance to come against the Covenant. After securing it, the ODSTs worked to make it a strategically defendable point for the UNSC survivors. It was christened Alpha Base, and Major Silva ordered all remaining UNSC forces to rendezvous there to make a stand. The Covenant launched a large vehicular raid on Alpha Base of nearly a hundred Ghosts, but the 200 or so remaining ODSTs were able to repulse the poorly led assault.

The Covenant placed as high a priority on recovering UNSC life pods as the UNSC forces did. Many of the successful landings were soon sieged by Covenant attacks in an effort to minimize the number of UNSC personnel on the ring world. Individuals and teams such as SPARTAN-117's group spent a considerable amount of time securing life pods throughout the installation. However, the human forces had realized that Captain Jacob Keyes had been captured by the Covenant.

Truth and Reconciliation
SPARTAN-117 led Fire Team Charlie to rescue Keyes from the grounded Covenant Truth and Reconciliation. They would set down a short distance from the Covenant battlecruiser, and then infiltrate through its Gravity Lift. From there, they would find Keyes, rescue him, and then evacuate.

Surprisingly, the plan went quite well. Despite a pair of Hunters being deployed at the Gravity Lift, the approach was simple. From there, they were able to go up the Gravity Lift into the belly of the Truth and Reconciliation. The UNSC forces fiercely battled their way through alien corridors for fifteen minutes before reaching a spacious hangar bay. However, before they could continue, the Covenant were able to lock the doors on the Marines and the Master Chief, continually sending in waves of reinforcements to kill the trapped UNSC soldiers in the locked hangar bay. After losing a significant amount of soldiers, Cortana was able to use her computer countermeasures to force open a door that lead from the hangar to the bridge of the warship. SPARTAN-117 ordered his Marine comrades to wait in the bridge while he assaulted a Covenant brig, freeing Keyes and also three other captured Marines. However, by the time they returned to the bridge, Energy Sword-wielding Elites had slain the rest of the rescue team. They headed to the Hanger Bay for evacuation, but Covenant air patrols prevented their rescue. Instead, the team chose to hijack an enemy Spirit dropship that was laying dormant in the hangar bay, with Captain Keyes freed and secured.

Shortly afterwards, the UNSC Marine forces stationed at Alpha Base, under the judgment of Major Silva, decide to embark on a risky raid on the crashed hulk of the Pillar of Autumn to get much-needed supplies and ammunition, vehicular support, and fuel for the ferrying Pelican Drop ships. A small 62-man ODST force under the leadership of Lieutenant McKay raided the Covenant-patrolled crash site of the Pillar of Autumn, using sniper fire to eliminate most of the defending Ghost attack vehicles and infantry, and then securing the ship with 10 casualties, securing much-needed supplies for the UNSC forces. However, the Covenant learned about this, and before McKay's force can return to Alpha Base and drop off the newly-seized supplies and vehicles that they had secured from the wreck of the Pillar of Autumn, they were intercepted by a superior Covenant force in the rocky terrain. The tide of battle is turned to the UNSC's favor, however, by discipline, organization, and luck. The UNSC's four Scorpion Tanks used shell bombardment to destroy the attacking Covenant Wraiths, while the UNSC Warthogs fought off waves of assailing Banshee attack craft and Ghosts. The Covenant's aggressive tactics yielded no advantage, and the well-coordinated UNSC rocket units and a particularly bold feint by Second Lieutenant Oros destroyed the dozens of Covenant Banshees, and Marine sharpshooter Lance Corporal Jones took a serendipitous sniper rifle shot, killing a Covenant Elite officer, disorienting and fragmenting the Covenant resistance as UNSC forces plowed through the Covenant remnants. .

The Flood
"Ahh... augh! Get it off! Get it-augh!"

- Private Riley, first UNSC casualty to the Flood

Meanwhile, a Covenant Elite, 'Qualomee, surrendered to the UNSC and yielded intelligence about a Covenant arms cache in a faraway swamp. After Captain Keyes meets Major Silva and congratulates him and his ODSTs for establishing Alpha Base for the UNSC, Keyes decides to lead a platoon of Marines under the leadership of Staff Sergeant Avery J. Johnson to secure this Covenant arms cache while SPARTAN-117 is ordered to a remote island where a Forerunner construct named the "Silent Cartographer" is stationed - a construct that is believed to hold an entire and fully-detailed map of Installation 04. Securing the intelligence would give the UNSC an inestimable edge in their resistance efforts. Two Pelican Drop ships inserted two squads of Marines (lead by Gunnery Sergeant Waller and Sergeant Stacker) and SPARTAN-117 to an edge of the island. They were deployed under fire, but managed to secure a beachhead and a landing zone for the UNSC to land further reinforcements (including an M12 Warthog LRV). SPARTAN-117 and two Marines, PFC Hosky and an unnamed Marine corporal, took the Warthog, driving it to a cliff that was the entrance to the facility that housed the Silent Cartographer. However, the Covenant knew that the UNSC was attempting to secure the intelligence, and locked the entranceway, forcing the trio to make a detour to a security substation, deactivating the locked door before finally being able to tunnel their way through the Covenant defenses stationed inside the subterranean facility and retrieving the intelligence.

The surviving Marines still awaiting on the landing zone were scheduled to be reinforced by Pelican Bravo 22 when it was shot down by ground fire. Shortly afterwards, a pair of Spirit drop ships converged on the Marines by the landing zone, overwhelming them on the exposed terrain of the beachhead while SPARTAN-117 was still within the bowels of the underground facility. The forces on the beachhead are presumed dead. The lack of a UNSC-held landing zone on the surface of the island meant that Echo 419 could not extract SPARTAN-117 and his vital intelligence. However, Cortana finds that there is a massively honeycombed system of massive open corridors that permeate the ground of Installation 04, allowing a vehicle that entered the system to travel essentially anywhere undetected. Echo 419 is able to slip into the system and rescue SPARTAN-117. Using this system, SPARTAN-117 was able to be transported to the installation's Control Room.

Captain Keyes's team, meanwhile, landed near the site of the supposed "weapons cache". The two Marine teams cleared the Covenant defenses and began to search the underground facility where the arms were supposedly stored. One team stayed back while the other team, led by Staff Sergeant Johnson, searched the facility. Eventually, they ran across a door that had been electronically locked by the Covenant. Keyes ordered the door be opened. Shortly after entering the room, however, the advance team, including Captain Keyes, was attacked by parasitic organisms called Flood. Staff Sergeant Johnson managed to escape after the Flood Infection Forms took over his fellow Marines. Supposedly his body was unsuited for infection due to the affects of Boren's Syndrome. Private Bisenti would also survive the attack. Due to a failed infection attempt, Private First Class Jenkins was infected but had control over his consciousness and some body functions. Jenkins was later captured and interrogated by UNSC forces.

Cortana had already learned of the threat the Flood was and sent SPARTAN-117 to save Keyes. However, the Spartan would only find Jenkins' helmet and it's mission log. When he finally evacuated, he ran into a surviving squad of Marines and began retreating with them to a nearby structure, whilst being assaulted by Flood Combat forms. After securing an LZ for dropship Echo 419, he would then disappear after encountering 343 Guilty Spark. .

Meanwhile, Alpha Base came under heavy attack from a Covenant force. The Covenant had identified that SPARTAN-117 was responsible for the majority of the hundreds of casualties during the raid on the Truth and Reconciliation, and an ambitious Special Operations Elite named Zuka 'Zamamee was tasked with leading a platoon of Special Operations Elites into the bowel of Alpha Base to assassinate the SPARTAN-II while a much larger Covenant force distracted the UNSC defenders. Using a captured Pelican drop ship, Charlie 217, 'Zamamee infiltrates the UNSC HQ, but SPARTAN-117 is nowhere to be found. Meanwhile, Spirit drop ships hit the UNSC defenses, but are thrown back by massive amounts sniper rifle fire and several Scorpion Tanks. Furthermore, the ODSTs organize several hunter-killer teams to re-secure the HQ, murdering all the Special Operations Elites save 'Zamamee, who is able to rescue a UNSC-captured Grunt named Yayap and escape with Yayap in a Banshee fighter.

Containment
While SPARTAN-117 journeyed through various areas in Installation 04, Cortana was able to briefly contact Alpha Base, informing them that SPARTAN-117 will use the Pillar of Autumn's fusion reactors to trigger a thermonuclear explosion that would destroy Installation 04. However, the explosion would kill all life on the ring world if the UNSC forces failed to escape before the detonation. Major Silva came up with an daring plan: to recapture the damaged Truth and Reconciliation, already taken to launch readiness by a Covenant repair team, and to use it to escape the ring while SPARTAN-117 sets off the delayed thermonuclear reaction and escapes as well. Afterwards, SPARTAN-117 will rendezvous with the UNSC-controlled Truth and Reconciliation and then they will return to Earth, heralding a glorious victory for the UNSC: a superweapon destroyed, a Covenant armada annihilated, and the first Covenant warship ever captured.

However, this plan was not perfect. Alpha Base's contingent learned of the Flood as a brief skirmish with nearby Covenant patrols leads to an exaggerated battle as the Flood attack, and the UNSC personnel are mostly horrified by the Flood's combat potential and viciousness. After it was learned that Flood are aboard the Truth and Reconciliation, ODST Lieutenant McKay and UNSC AI Wellsley were convinced that the UNSC survivors should not risk severe casualties boarding a Flood-held starship...and even if they prevail and bring the warship to Earth, if any Flood managed to invade Earth, it would "fall". However, Silva, the commanding officer of all of Alpha Base's forces after Keyes's tragic death, demurs, and instead assaults the Truth and Reconciliation, with his ODST special forces operators securing the ship with minimal casualties and the non-combat personnel in tow. Surviving senior naval officers from the Pillar of Autumn are sent in after the ODSTs deem the warship as "secure", and they move swiftly to undersee last-minute repairs to the Truth and Reconciliation before it takes off.

However, McKay still holds doubt - even if the Truth and Reconciliation took off successfully and reached Earth without incidence, the pandemiological threat of the Flood meant that if even one parasite managed to infect a human on Earth, it would be a disaster of a galactic scale. McKay, in conscience, is forced to damage the Truth and Reconciliation's reactors and destroy the warship, killing herself and all 312 UNSC personnel and surviving Flood beings on board, thereby destroying nearly all of the UNSC presence on the ring.

Escape
While almost all of Alpha Base's personnel are eliminated in the tragic self-detonation, Avery J. Johnson did not make it onto the Truth and Reconciliation after surviving the Flood attack. He was able to join three other UNSC personnel that were, likewise, not aboard the Truth and Reconciliation as well: Office of Naval Intelligence Lieutenant Elias Haverson, ODST Corporal Locklear, and Warrant Officer Sheila Polaski. Together, the four escaped Installation 04 in a Pelican Dropship before SPARTAN-117 set off the thermonuclear detonation, himself escaping in a C709 Longsword-class Starfighter.

After Installation 04's destruction, Cortana conducted a scan of the interstellar debris of Installation 04, and found an echo of Polaski's dropship. SPARTAN-117 was able to link up with the four other UNSC survivors of the cataclysm, but seven Covenant warships: six Covenant Cruisers and one 3-kilometer Covenant Flagship, entered the system and opened fire at the fighter and the Pelican. SPARTAN-117 ordered everyone into the Longsword while the Pelican was sent ahead as bait, drawing plasma fire from the Covenant starships while Cortana remotely hacked into the Covenant task force's Battlenet, causing electronic mayhem that temporarily stunned the weapons aboard the warships while the Longsword, with SPARTAN-117, Johnson, Locklear, Polaski, and Haverson on board, slipped into the hangar of the Covenant Flagship. The small group of unprepared UNSC warriors fight through Elite defenders while Cortana slips into the Covenant Flagship's computer systems from a fiber optic port, and then proceeds to drain atmosphere from most segments of the ship, asphyxiating most of the Covenant guards. However, the bridge's crew still remains oxygenated, and seal the doors to the bridge, barricading themselves from the UNSC attackers. This measure is futile: SPARTAN-117 uses a plasma grenade to electromagnetically scramble the locks to the doors, and the UNSC forces seize the understaffed bridge with minor difficulties...SPARTAN-117's personal energy shields fail just as an Energy Sword-wielding Zealot appears. SPARTAN-117 has his arm nearly amputated in the resulting wrestle, but Marines Johnson and Locklear are able to severely wound the Zealot, before throwing him into an escape pod and launching it.

Cortana interfaces with the bridge's navigational controls to let the now UNSC-controlled flagship, Ascendant Justice, flee from the system, but a Covenant AI counters her, physically disabling the Slipspace drive of the Ascendant Justice and preventing Cortana from executing a jump out of the system before the six other Covenant Cruisers vaporize the flagship. SPARTAN-117, however, makes a journey through the damaged warship to kill the Grunts guarding the disabled Slipspace drive, and then forces a Covenant Engineer to repair the drive, allowing Cortana to program the Ascendant Justice to jump from the system, just barely surviving the bombardment of the six Covenant Cruisers, thus ending the UNSC retreat from the war torn system.

First Battle of Earth


"We're taking this fight to the surface."

- Commander Miranda Keyes

The UNSC Marine Corps remained in large force on Earth and its geosynchronous Orbital Defense Platforms, and were thus mostly prepared to repulse a Covenant invasion when fifteen Covenant capital warships, two Assault Carriers and thirteen Covenant Cruisers, entered the Sol System.

Because the large number of orbital Magnetic Accelerator Cannons made it almost impossible for any of the Covenant warships to reach Earth unscathed, the Covenant sent boarding craft to board numerous Orbital Defense Platforms. The boarders swiftly penetrated the platform's infantry defenders and set bombs in the core of the stations and then detonated them, destroying the defense station inside-out with minimal losses. Malta Station and Athens Station fell almost immediately to the internal sabotaging,

Covenant forces invaded Cairo Station as well, the command center for UNSC fleet operations during the battle. The crew of the Cairo held out longer, albeit with heavy casualties, and Marines and crewmen fought a coordinated holding action, trying to force the boarders out of the peripheral hangars and the intermediate common rooms, but their defenses were penetrated by a small force of Elites and Grunts that planted a bomb in the Fire Control Center of Cairo Station. Only SPARTAN-117's timely reinforcement of the Marine garrison and his assault on the Fire Control Center managed to disarm the bomb in time, thus saving Cairo Station from destruction.

However, despite the fact that Cairo Station and hundreds of others of Orbital Defense Platforms still stood, the loss of the Malta and Athens allowed a Covenant Assault Carrier to penetrate the UNSC Navy's defensive web around Earth, and this Assault Carrier attacked Mombasa, Africa.

However, the 405th Marine Infantry Division, some of whose elements were stationed in Mombasa, Kenya at the time, were able to battle against the Covenant as the Assault Carrier landed Phantoms, Orbital Insertion Pods, and used its Gravity Lift to directly deploy reinforcements. Furthermore, the was also able to use its Pelicans to deploy Marines, vehicles, and equipment to assist the 405th Marine Division.

The In Amber Clad sent three Pelican Dropships to reinforce and to drop off Marine and ODST fire teams at Grid Kilo 23, where SPARTAN-117 would lead a coordinated Marine assault to liberate the Covenant-contested city center, when the Pelicans were unexpectedly attacked by a Scarab walker en route to their landing zone. Two of the Pelicans were forced to break off when one of the In Amber Clad's Pelicans, which contained SPARTAN-117 and Sergeant Johnson, crashed into a building.

Miraculously, everyone aboard the Pelican save the pilot survived, and SPARTAN-117 was able to rally the Marine passengers, holding a nearby battle-damaged building against multiple Covenant attacks until the Marines could be evacuated at a nearby clearing.

SPARTAN-117 meanwhile proceeded to liberate a small ODST force, under Gunnery Sergeant Pete Stacker, at Hotel Zanzibar, and then fought through a Covenant-controlled tunneled highway.

Meanwhile, the Scarab brushed aside Marine defenses, walking over the highway, and then past a large bridge and into the midsection's of the city completely unopposed by Marine small arms fire.

SPARTAN-117 and a small team of Marines pursued the lumbering walker, fighting through long segments of Covenant-controlled city locations until reaching the city center, where "A" Company of the 77th Marine Regiment defended against waves of Covenant Ghosts, Wraiths, and Phantoms in a fortified industrial building.

Completely undeterred by UNSC Marine Corps Scorpion Tanks or machine guns, the Scarab too brushed aside "A" Company in its fortified building, until it was caught in a nearby canal system and physically trapped. Pelicans commenced air strikes on the trapped walker, but the armor was barely scarred, and there was no other choice but to board the Scarab itself and destroy internally. While "A" Company's remnants provided suppressive fire, SPARTAN-117 jumped aboard the enemy walker, killing its crew and destroying it.

Battle of Installation 05
"Cortana? What exactly am I looking at?"

- Commander Miranda Keyes

"That...is another Halo."

- Cortana

"Say what?!"

- Sergeant Major Avery J. Johnson



After the Slipspace cataclysm over Mombasa involving the Prophet of Regret's Assault Carrier, pursued the Prophet of Regret's carrier through Slipspace to Installation 05 (Delta Halo), where Regret was determined to sermonize and then activate the ring world and bring about The Great Journey to all of the Covenant and lead them to a believed transcendence.

While the carrier sent Phantom drop ships to the surface to land the Covenant Army in the immediate area and to allow Regret to begin his religious ceremony in a Forerunner temple before activating the ring, the In Amber Clad passed over the approximate position where Regret's temple was, deploying ten Human Entry Vehicles, and shortly afterwards, a flight of Pelican Drop ships.

While only a handful of HEV pods survived the insertion, the surviving ones contained SPARTAN-117 and several heavily armed Orbital Drop Shock Troopers. Together, they cleared a small area of Shielded Plasma Cannons and a small group of Covenant soldiers, establishing an LZ for a Pelican Drop ship to deploy an M12 Warthog LRV and another pair of ODST operators.

SPARTAN-117 took the Warthog, and with the help of the ODSTs, fought through a Covenant-controlled bridge before another Pelican deployed a M808B Scorpion MBT that provided extra firepower for a UNSC offensive to push through two large Forerunner temple ruins, where yet a third Pelican from the In Amber Clad dropped off weapon resupplying drop pods and Marine reinforcements.

After battling through a ravine entrenched by aerial Drones and Jackal Snipers, the UNSC force reached a small Forerunner temple, where SPARTAN-117 and two remaining Marines fought off a pair of Honor Guard Elites before continuing onwards through a series of closed temples before reaching an open circular platform where a fourth Pelican was scheduled to air-drop additional armaments.

However, before that could happen, SPARTAN-117 and whatever surviving Marines were attacked by an airborne Phantom dropship that deployed two heavy Hunter warriors that savagely assailed the UNSC personnel before they were fought off and the fourth Pelican arrived to bring BR55 Battle Rifles, Sniper Rifles, and M19 SSM Rocket Launchers. SPARTAN-117 continued to take a gondola from the open platform, fighting off enemy Banshee fighters and Ranger Elites to reach a series of submerged temple structures and finally another gondola to the temple where Regret was sermonizing.

SPARTAN-117 and his Marines began fighting off Energy Sword-armed Honor Guard Elites, managed to assassinate the Prophet of Regret before the Prophet of Truth and Prophet of Mercy ordered an Assault Carrier to use an Energy Projector to vaporize the temple in a vain attempt to kill the "Demon", afterwards which contact was lost with SPARTAN-117 and whatever remaining Marines that were accompanying him at the time.

Meanwhile, at High Charity, the Changing of the Guard occurred, and the Arbiter was deployed to the Sentinel Wall to deactivate the containment shield that prevented any being from journeying to the Library and retrieving the Index. After the Containment Shield was deactivated, apparently a company-sized Marine unit was sent to the Flood-Infested Wall to try to make their way to the Library, but were overwhelmed by the Flood and assimilated.

However, while the Arbiter punched through heavy Flood defenses surrounding the Library, the UNSC tried again to attack the Library and secure the Index to ensure that the Covenant would not use it to activate Installation 05 and cleanse the galaxy of sentient life in their misguided religious quest. While the Arbiter cleaned out the Flood infestation barring way to the Library, the UNSC were able to slip a small force of Marines directly next to the Forerunner structure, who managed to infiltrate it, but took casualties from Flood infection forms.

At the end, only Miranda Keyes and Avery Johnson survived, but while Commander Keyes secured the Index, the Arbiter managed to race into the Library, incapacitating Perez and MacKenzie before launching a sneak attack on Johnson and Keyes, knocking them both unconscious before securing the Index for the Covenant.

Battle of High Charity
However, SPARTAN-117 had survived the Covenant vaporization of Regret's Temple, but had a more insidious fate: deceived by the Flood overlord creature known as the Gravemind, he became the Flood's pawn, being teleported to High Charity in an attempt to battle through legions of Covenant troops to capture or kill the Prophet of Truth and secure the Index to ensure that Truth would not activate Halo.

Early on during the Battle of High Charity, however, Cortana detected the CNI wave signatures of several UNSC Marines inside the bowels of High Charity. SPARTAN-117 took a short detour, launching an attack on a four-story Covenant detention facility and rescuing five Marine prisoners from a pair of heavily guarded Covenant detention blocks, who aided him in his battles through High Charity.

In the end, however, none of the Marines survived, and the Prophet of Truth escaped from SPARTAN-117's grasp, fleeing in a Phantom drop ship while Tartarus, Chieftain of the Brutes, took the imprisoned Keyes and Johnson, along with a cohort of Honor Guard Brutes, in two separate Phantoms to Installation 05, where Tartarus planned to use the Index to activate Halo at the Control Room of Installation 05.

Meanwhile, the In Amber Clad had not fared well after Keyes's team's departure from the frigate. Flood forces managed to storm aboard by unknown means, killing the human crew and using their unprecedented sentience to crew the ship. They managed to crash it onto a high tower on High Charity, spreading Flood into the Covenant planetoid. With the Pelican dropships which provided the Flood with extensive routes to invade High Charity.

A number of its crew were assimilated into the insular of the parasite, fighting against SPARTAN-117 as the Master Chief battled through a Flood-infested High Charity to secure access to the Forerunner Ship.

While Tartarus took the Index and Miranda Keyes in one Phantom to the Control Room, the other Phantom, carrying Johnson, diverted its route...when Johnson apparently managed to overpower the Brute crew and escape the Phantom, managing to slip through to an inactive Scarab walker while two nearby Marines, most likely Sergeant Stacker and Sergeant Banks, fought off a large pack of Brute Captains with Plasma Pistols. These two Marines were saved when the Arbiter arrived, slaying the Brutes.

However, Johnson pointed the Scarab's main cannon at the Arbiter...but not in bad will. The Arbiter and Johnson made a truce. Johnson would use the main cannon of the Scarab to punch a hole in the surface of the Control Room while the Arbiter would charge in with a Banshee and defeat Brute Captains and Brute Honor Guards before confronting Tartarus while Johnson would unmount from the Scarab, joining a large number of Covenant Separatist Councilors, Zealots, and Special Operation Elites in aiding the Arbiter to kill Tartarus and prevent him from using the Index to activate Halo.

However, Tartarus put up a significant resistance, and despite being heavily outnumbered by the Arbiter's Elite comrades, used his personal energy shield and the gargantuan Fist of Rukt to defend himself. In the end, however, Tartarus was unsuccessful.

Johnson used his exceptional marksmanship with a sniper rifle to take down Tartarus's powerful shields from afar with a Particle Beam Rifle while the Arbiter and his allies attacked the unshielded Tartarus, finally killing him and bringing an end to the Chieftain of the Brutes and preventing the firing of the Halos, terminating the UNSC Marine Corps's involvement at Installation 05.

Second Battle of Earth


The UNSC Marine Corps had a substantial involvement in the defense of Earth during the Second Battle of Earth, being the majority of the UNSC's ground defenses when the Covenant landed on Earth in multiple locations, which included the continent of Africa.

A scattered Marine presence appeared to remain in force in the immediate vicinity of New Mombasa, where a Slipspace disaster wiped off a significant number of Marine assets, including at least a portion of the 405th Marine Infantry Division. Marine forces managed to hold out in an underground base, waging a guerrilla war against the Covenant occupiers of East Africa for some time before the Master Chief's arrival from on board the Forerunner Dreadnaught. After his recovery, Marine forces prepared to launch an offensive against the Covenant and reach what they thought to be the Ark. Unfortunately, the Covenant found them first, and assaulted the base with heavy numbers of infantry. The Marines, Master Chief, and the Arbiter managed to fend them off long enough to evacuate the remainder of the survivors, and destroyed the base rather than let the Covenant learn of their plan - the Master Chief personally led a small number of Marines along the Tsavo Highway, penetrating through numerous blockades and passing patrols of Choppers and Wraiths, eventually reaching the town of Voi. Once at Voi, the Master Chief and the marines began their operation - destroying anti-air emplacements to allow Pelican drop ships to insert more UNSC forces, including the Arbiter. One emplacement from a perimeter of heavy Anti-Air Artillery was disabled, allowing UNSC Longsword Fighters and Frigates to attack the Forerunner Dreadnaught. The Prophet of Truth, however, activated a portal over the "ark," leading to the true facility - and a Covenant Cruiser made a pinpoint slipspace jump in-atmosphere above Voi, infected by the Flood The Marine Lieutenant commanding forces in Voi soon was forced into a brutal firefight against the Flood, in which he and all of the Marines in his command post were infected, save for a single survivor.

Spartan-117, having destroyed the AA turret, reentered Voi with the Arbiter, and began attacking the Flood and rescuing Marine survivors. Despite his best efforts, only five Marines, not counting the traumatized survivor of the raid on the CP, were rescued. About midway to the crash site of the Flood-controlled ship, the unlikely rescuers of the humans, the First Sangheilian Fleet under Rtas 'Vadum, arrived, and deployed Special Operations Sangheili to assist in the cleansing of the city. The Marines that the Chief had rescued fell back, and were transported to the Forward Unto Dawn. This ended the UNSC Marine Corps' involvement at Earth.

Exploration and Touchdown
"All armor form up, hit 'em where it hurts!"

- Gunnery Sergeant Pete Stacker, leading a Scorpion platoon on the Ark

Following negotiations between Admiral Hood and Rtas 'Vadum onboard the Sangheili flagship Shadow of Intent, it was agreed that the Sangheili, plus the Forward Unto Dawn and her Marine complement, would follow Truth through the Portal to the Ark.

Upon emerging from the Portal, the Sangheili fleet encountered a Jiralhanae fleet three times their size. With 'Vadum's ships holding the line against Truth's fleet, Spartan-117, the Arbiter, and a combined Marine-ODST task force under Sergeants Avery Johnson, Reynolds, and Pete Stacker landed on the Ark.

Spartan-117's first objective was to clear AA defences so Commander Keyes could set down the Dawn; he was dropped off with four ODSTs near the first Covenant battery. They proceeded to knock it out, and linked up with two Marines whose Pelican had been shot down. The impromptu assault team hijacked a Jiralhanae Prowler, commandeered a Mongoose from the wreck of the Pelican, and headed out to the last AA battery. Once it was destroyed, Commander Keyes set down the dawn, and deployed Bravo Team, an armored of four Scorpions under Gunnery Sergeant Stacker, plus 343 Guilty Spark.

The armored platoon forged ahead, and managed to penetrate a wall seperating Spartan-117 from his objective: a Cartographer similar to the one found on Installation 04. However, a Scarab, a Phantom, and several Wraith tanks were in the way. Bravo Team managed to skillfully bludgeon their way through the Covenant defences, and Spartan-117 managed to arrive at the Cartographer building, discovering that Truth was holed up in a Forerunner Citadel in the center of the Ark.

Pathfinding and Confrontation
Upon the discovery of Truth's location, a flight of five Pelicans and two Separatist Phantoms made way to the Citadel. They split up into three groups; the Arbiter led his two Phantoms' worth of Elites, Avery Johnson led a duo of Pelicans filled with Marines, and the Master Chief took another two Pelicans holding ODSTs. Their goal was to split up and quickly deactivate the three shield generators surrounding the Citadel. Miranda Keyes held back and waited to give the Master Chief's team support, fearful of the Anti-Air Wraith guarding their insertion point. Her fears were justified when one of the two Pelicans of the Master Chief's team was shot down. The remaining bird landed the Chief and just three ODSTs to take out the Brute position and disable the Wraith. Through much hardship and desperation they did just that, and awaited Miranda Keyes' reinforcements of three more Marines, a Warthog and a Mongoose. The mounted strike force then fought their way to the interior of the island, towards one of the three generators the battle group was tasked with taking out.

The Master Chief and his team proceeded inside the generator complex, fighting a myriad of loyalist Covenant along the way. The Chief made his way up to the control center, slew a pack of Brutes including a Brute Chieftan, and deactivated the generator. Approximately that same moment, the Arbiter and his Elites deactivated their generator as well. Expecting the third station to fall along with them, Miranda and the Chief were shocked to hear that Johnson's team of Marines was being overwhelmed by Brute reinforcements and were forced to fall back. Keyes rallied the other two strike teams and ordered them to finish the job that Johnson's team had started.

The team being led by the Master Chief sped back down to their original landing zone, and were picked up by a flight of Hornets. Assisted by the remaining two Pelicans, they all battled their way through stiff Covenant air resistance and saw the massive enemy force that had understandingly subdued the now apparently vanished Johnson's team. A vicious landing operation commenced, and the whole cliff side station was violently cleared for the Master Chief and remaining Elites (including the Arbiter)to storm the generator complex and deactivate the final power source.

As the shield went down, a fully flood-consumed High Charity burst out of slip-space (presumably from the portal on earth) onto the Ark. A number of pieces of debris smashed through the Elites cruiser, severely damaging it. A chunk of infested debris also smashed through the complex, and the Chief fought the flood as he got back outside to the deployment zone.

Upon her arrival, Keyes had delivered a Scorpion, a Gauss Warthog, and a few Mongooses for him to use as he led the remaining Marine ground force and a handful of Elites to the interior of the island and begin the siege of the Citadel. The mounted and heavily armed task force battled their way down the Covenant fortified cliff side and were then met by a terrifying welcome party of two Scarab tanks. The remaining airborne force of Pelicans and Hornets assisted the Chief's team as they fiercely fought and destroyed this last line of defense.

The Master Chief and the Arbiter then proceeded into the Citadel to rescue Johnson and kill the Prophet of Truth, but only made it after Miranda Keyes' desparate rescue attempt. Johnson escaped using Miranda's Pelican and returned to Forward Unto Dawn, and the Master Chief and the Arbiter had to fight their way out, escaping the Flood and the Covenant. The remaining marines and elites were withdrawn to the Shadow of Intent as the Master Chief ventured into the remains of High Charity in search of Cortana.

Later, Avery Johnson's exploits aided in the final destruction of the reborn Installation 04 and the Flood, giving his life for Humanity and the Arbiter along the way.

As Forward Unto Dawn crash-landed into water mass on Earth, marines (likely those that did not travel to the Ark, or had remained behind with Lord Hood) helped the Arbiter out of the wreck, and also participated in the memorial ceremony near the Ark Portal. It is likely that Marines were involved in peacekeeping and aid missions on Earth, or sent into space to battle against and remaining Covenant Loyalist forces.

Armor


Marines have few bio-augmentations and usually wear passively camouflaged armored uniforms. They wear dark-colored metal-alloy armor to shield them from gunfire. Marine helmets are mounted with display holographic text-displaying eyepieces and helmet recorders. As well as helmets, Marines on field operations in more humid locales sometimes choose to wear boonie covers, or those in colder environments don visored and thicker helmets.

Marine NCOs are easily recognized by the hat that has the Eagle, Globe and Anchor of the Marine Corps that they wear. They often times substitute this command hat for a protective helmet. While they occasionally wear helmets in Halo 2, in Halo: Combat Evolved, they are portrayed as always wearing the hat. Sergeants are (usually) more tactically adept, are better shots, and are tougher to kill than normal Marines, though these differences in game play, if any, are largely unnoticed.

In Halo: Combat Evolved, Marines sported gray fatigues, and over that, pieces of brown armor plating covering the head, shoulder, and shins. At times, they only wore their fatigues, even forgoing helmets or even body armor plating in hazardous combat operations. At times, their helmets were also substituted for boonie covers and sometimes bandanas. Some Marines had small backpacks built into their back chest armor plating, although these backpacks appeared to be part of a medical uniform, as the text "EMP" (Emergency Medical Pack) was written on the back.

The UNSC Marine armor was much different in Halo 2. Marines appear to wear an armored suit underneath jungle camouflage-patterned padding, and a conspicuous green load-bearing armored vest over their chest padding for additional protection. The standard-issue armor is more efficient than the body armor issued to the Marines of Halo: Combat Evolved. The Marines appear as less of a rag-tag group in Halo 2 than they were in Halo: Combat Evolved. They no longer wear boonie covers and are never in combat without their armor. In fact, in Halo 2, the only times Marines are seen without any armor is during the awards ceremony aboard Cairo Station during a noncombat situation. In that case, they were wearing their dress whites.

In Halo 3, the Marines sport armor that is an evolution from that of Halo 2, with heavier and well-articulated woodland-colored full body armor (chest plate, back plate, shoulder plates, elbow pads, and knee pads) over camouflaged fatigues, allowing them to passively camouflage well in jungle terrain. Marines sometimes don boom mikes or black-phased sunglasses on their helmets, and others carry backpacks.

Orbital Drop Shock Troopers, otherwise known as "Helljumpers", are a special operations group within the UNSC Marine Corps. They wear full body armor with a dark black camouflage scheme. They sport heavy and articulated black body suits, which is also believed to be pressure armor that allows for safe transport in small ships in turbulent atmospheric entry and also fully-covering and fully-visored black helmets.

Marines operating in cold environments wear environmentally-tailored armor, with snow camouflage scheme heavily thickened and fully-covering body armor and visored helmets with balaclavas.

It should be noted that all marines have a HUD not just from their helmets, but from their standard neural interface.

Infantry Weapons
The standard armament for UNSC Marines is the MA5 series of assault rifles, with most Marines being issued with the MA5B variant until its replacement in late 2552 by the MA5C variant. Suppressive fire was provided by the M247 GPMG during the First Battle of Earth, presumably replaced by the AIE-486H Heavy Machinegun by the time of the Second Battle of Earth. Special Forces and vehicle crews are issued to M7 and M7S Submachine Gun variants, with personal sidearms filled by the M6 Series.

Designated Marksmen are issued with the BR55 Battle Rifle and its successor, the BR55HB SR. Snipers operate the SRS 99C-S2 AM and SRS 99D-S2 AM sniper rifle variants for anti-materiel and anti-personnel operations, with the M99 Special Application Scoped Rifle used as a dedicated anti-materiel weapon.

Anti-armor fire is provided by the M19 and M41 rocket or missile launchers and the W/AV M6 G/GNR Spartan Laser directed-energy-weapon. Anti-air fire is provided by the M19-B SAM Missile Launcher, capable of destroying Banshee's and damaging Seraph fighters.

Ground Vehicles
The UNSC Marines standard vehicle is the M12 Warthog LRV, used for armored patrols and personnel mobility. The M12G1 Warthog LAAV is used as an anti-armor vehicle, mounted with an M68 Gauss Cannon, as well as the M12A1 Warthog LAAV mounted with a 102mm SC-HE Rocket Turret, firing 102mm shaped-charge high-explosive rockets. For heavy combat, the Marine Corps operates the M808B Scorpion Main battle Tank, and uses the Wolverine in an anti-air role.

Aircraft
The UNSC Marine Corps' most common aircraft is the Pelican dropship, either the D77-TC or, in late 2552, the D77H-TCI variants, used as multirole aerodynes, able to carry a mixture of Scorpion anti-tank rockets and Anvil II air-to-surface missiles, and equipped with chin mounted 7.62mm machineguns or 70mm autocannons. The Corps also operates the Longsword as it's primary fighter, using it too full effectiveness in Air-To-Air, Air-To-Ground and in Space Combat, the Sparrowhawk as an air-to-ground attack gunship, the Shortsword short-range bomber, the Hornet aerial support vehicle, and the SkyHawk VTOL strike fighter.

Marine air forces are intended usually to support ground forces, either in close-air-support or air interdiction roles, with the UNSC Navy taking on the dedicated air force role.

Halo: Combat Evolved

 * MA5B ICWS Assault Rifle
 * SRS99C-S2 AM Sniper Rifle
 * M90 Assault Shotgun
 * M6D Pistol
 * M9 HE-DP Grenade
 * M19 SSM Rocket Launcher

Halo 2

 * M6C Magnum Sidearm
 * BR55 Battle Rifle
 * M7/Caseless Sub Machine Gun
 * SRS99C-S2 AMB Sniper Rifle
 * M41 SSR MAV/AW Rocket Launcher
 * M90 Assault Shotgun
 * M9 HE-DP Grenade
 * M247 GP Machine Gun

Halo 3

 * M6G Personal Defense Weapon System
 * M7/Caseless Sub Machine Gun
 * MA5C ICWS Assault Rifle (standard issue)
 * BR55HB SR Battle Rifle
 * M90A Close Assault Weapon System
 * SRS99D-S2 AM Sniper Rifle
 * M41 SSR MAV/AW Rocket Launcher
 * W/AV M6 Grindell/Galileian Nonlinear Rifle "Spartan Laser"
 * M9 HE-DP Grenade
 * M7057 Defoliant Projector

Halo Wars

 * BR55HB SR Battle Rifle
 * SRS99C-S2 AM Sniper Rifle

Halo: Combat Evolved

 * Ghost
 * M12 Warthog LRV
 * M808B Scorpion Main Battle Tank
 * D77-TC Pelican

Halo 2

 * Ghost
 * M12 Warthog LRV
 * M12G1 Warthog LAAV
 * M808B Scorpion MBT
 * D77-TC Pelican
 * Albatross Heavy Dropship

Halo 3

 * M274 ULATV Mongoose
 * M12 LRV Warthog
 * M12G1 LAAV Warthog
 * M831 Troop Transport
 * M808 Scorpion MBT
 * Elephant
 * Hornet
 * D77H-TCI Pelican
 * Albatross Heavy Dropship

Halo Wars

 * M12 LRV Warthog
 * Cougar Armored Vehicle
 * Wolverine Missile Vehicle
 * M808 Scorpion MBT
 * Rhino (Unknown Tank Class)
 * D77H-TCI Pelican Dropship
 * Albatross Heavy Dropship

Miscellaneous

 * CH252 Helmet
 * M52B Body Armor
 * VZG7 Armored Boots
 * S90 Gas Mask
 * Spoofer
 * VX7 Nerve gas
 * PG000E Portable Electricity Generator

Halo: Combat Evolved
In Halo: Combat Evolved, almost all Marines carry MA5B Assault Rifles, though occasionally they can be seen with a Sniper Rifle or the Shotgun, or very rarely the Covenant Needler or Plasma Rifle when no UNSC armaments are available.

They are relatively accurate with these weapons, with a hit percentage of 50% of upwards, often firing in short bursts with the Assault Rifle, killing a Grunt in one or two bursts, knocking a Jackal's shield off position with repeated bursts, or cooperatively working with other Marines to take down an Elite. Marines also often utilize Fragmentation Grenades in combat. They don't seem to be able to use rocket launchers even though there are rocket launchers around the bodies of their dead.

Marines will strafe and avoid fire, but not usually seek cover. They will roll out of the way of a vehicle but they will not pick up different weapons, Health Packs, or additional ammunition. Marines are also good pathfinders, often able to maneuver around obstacles such as boulders or fallen crates in their paths.

Marines are conscious of fragicide. After you have killed two humans of any type, the rest of the Marines in the campaign level will attack the player. If one kills Captain Keyes, or any other members of the bridge crew, in the campaign level The Pillar of Autumn, Cortana will lock the doors and summon a squad of invincible Marines to destroy you.

Marines hitch rides on player-driven Warthogs and Scorpion Tanks but are only capable of piloting Ghosts by themselves, but they are not very skilled doing so, and will often run over you if you are not careful. They will exit Scorpions and warthogs when the player leaves the driver seat. This is most likely due to the fact that the marines in the Assault on the Control Room level are scripted to exit their vehicle once the player exits it. Normally, they will stay in a Warthog or Scorpion until they are ordered to get out. Practically, they are only seen driving by themselves in the cut scene of the Pillar of Autumn.

Interestingly, the enemy the Marines are most vulnerable to in Halo: Combat Evolved are the Infection Forms of the Flood. Once one is attached to a Marine, it cannot be shaken off and will kill the soldier. Marines are poor fighters against the Flood and will often take heavy if not total casualties fighting them.

They also go "berserk" when they have taken too much damage. When down to one last bit of health, they will scream something (such as: "I'll take you all on!") then rush in a straight line toward the nearest enemy and fire in extended bursts (5-7 round burst as compared to 2-3 round bursts). This is usually the last thing they ever do, as they are badly wounded by this time and the lack of any melee attack (or the use of full auto with an assault rifle) makes them very vulnerable in the close-range combat they so clearly want to engage the enemy in. If a "berserk" Marine survives a fight, they will calm down and simply return to normal.

Halo 2


In Halo 2, the Marines are intelligent in different ways than in Halo: Combat Evolved.

They are more adept with vehicles, and are able to drive the Warthog, the Ghost, the Wraith, the Spectre, and the Scorpion, albeit a bit clumsily. They can now melee sleeping Grunts or enemies with their backs turned, a vast improvement over the Halo: Combat Evolved Marines, who had to fire their weapons in such situations and ruin operational covertness.

However, their firearm prowess has decreased, and most Marines are unable to achieve greater than a 25%-50% hit percentage with semiautomatic weapons such as the Covenant Carbine, and are instead more effective with close-range weaponry such as the M7/Caseless Submachine Gun or with heavy weaponry such as the M19 Rocket Launcher or the Fuel Rod Cannon.

Unlike Spartans and Elites they don't have the new ability to dual wield. However dual magnums are sometimes found next to certain marines or additional weapons ex.ODST on Delta Halo with dual pistols.

Although their friendly fire incidents are decreased, contrary to popular belief, Marines do use fragmentation grenades in combat, albeit quite rarely, and are unaware of splash damage from heavy weapons, sometimes firing a rocket into a nearby wall and killing themselves in the operation.

The Marines are far more interactive, sometimes speaking to each other while in combat and wield a greater variety of weapons. The player can also easily switch weapons with the Marines to make sure that they are well armed or that the player is. As in Halo: Combat Evolved, the Marines are aware of backstabbing, and will attempt to kill you after you kill a few of their comrades. The number of Marines that can be killed before the others turn against you oddly seems to vary depending on the weapon used and the way they are shot.

For example, taking out three Marines with head shots usually won't make them see you as an enemy, but meleeing three Marines usually does. However, if the player progresses through the level and meets another group of marines a distance away from where the betrayal was performed, they will be marked as allies once again. Also, if the player does not harm any marines who are shooting at him/her for a certain amount of time the Marine(s) will reluctantly return to an allied condition.

Halo 3


Once again they can utilize the Grunts' weakness for naps by using stealth and meleeing them. They seek cover much more often and even put their back up to the wall. They throw grenades much more often, and with great accuracy, especially in higher difficulty levels. They will sometimes also display group tactics never seen in previous games, like huddling into a tight group and combining assault rifle fire while combating Drones. Even though the Marines are smarter in Halo 3 they occasionaly will make mistakes, such as running off a cliff while driving a vehicle, more or less like any new player would.

The player can still exchange weapons with them, though they won't accept Brute Shots, Gravity Hammers, Energy Swords, or Sentinel beams.

After killing two of them, any that are in the area will come to take you out. After a while (even if you are constantly killing them) they will see you as a friend again. Interestingly, their accuracy becomes 100% if betrayed, and if there are two or more remaining Marines they can end the Chief's life in a matter of seconds.

Their audio clips have also been vastly expanded, and profanity is used to indicate wounded status if the IWHBYD skull is activated.

Marines get noticeably more depressed and irritable after encounters with the Flood.

Marines are seen in larger groups in Halo 3. In previous games they were usually seen in groups of 4-6, or one squad. Now several squads can be seen at a time, and all are able to assist you.

Trivia

 * The equipment and armor used by UNSC Marines resembles the modern-day US Army's Future Force Warrior Program, and conceptually may have been inspired by it.
 * Bungie went to great lengths to ensure that the Marines reflected the diversity of Earth; some Marines look and sound American, while others appear Asian, African, Australian, Hispanic and Italian. Many were also modelled on members of Bungie's own staff members to provide further diversity.
 * In the Halo: Combat Evolved level "Truth and Reconciliation", Captain Keyes tells a Marine to "Stow your belly aching soldier, remember you're a leatherneck!" Leatherneck was a nickname earned by U.S. Marines in the 19th century.
 * In the Halo series, Marines will turn on the player if he/she begins killing allied units.
 * In 2007, Bungie hired an actual Marine, Christian Allen, who served with the US Marines from 1995 to 1999, as a Design Lead. Rather than a frontline combat unit, though, he served as a member of the US Marine Corps Military Police
 * In Halo: CE, shooting a dead Marine's armor with a ballistic firearm would eventually send a ricochet at your shields. Meleeing them would result in a loud CLACK! However, in Halo 2 and 3, the bullets do not ricochet, but penetrate the dead bodies and result in a large amount of blood.
 * In Halo: CE, Marines cannot see invisible Elites, even if they have bright, glowing energy swords.
 * In Halo 3, Marines generally tend to care greatly about the welfare of the Master Chief, and become scared and upset if you die. However, when playing Co-op with the Arbiter, if the Arbiter dies, the Marines don't seem to care at all and act as if it's unfortunate, but still unimportant.
 * UNSC Marines tend to perform as well when seriously injured, as they do in full-health condition; an unrealistic feature of combat. But this is to be expected, as very few games properly display combat weariness for obvious gameplay-related reasons.
 * In Halo 3, you can unload a full magazine onto the chest of a marine and they wont even flinch. Their armor really does protect them this time, but this is only in the center of the chest. You'll know it when you see it and the same goes for pilot helmets. Unload your entire gun on it and they wont feel a thing. Shooting the visor is a different story, shooting the marines in the helmet for halo 2 and 3 bare the same results: it will come off. If done with a battle rifle though with its 3 shot burst, the first shot will take it off and the other 2 shots will kill him. But there is a way to take it off with the BR without killing them.
 * In Halo 3, if you throw a plasma grenade at a Marine, it will stick to them. They will usually shout "Get it off me!" and dive into the other Marines.
 * In Halo 1, the faces of Marines are generated by building various portrait images into a form that can be "wrapped" around a 3D model.