UNSC Marine Corps/Gameplay
The UNSC Marine Corps and its personnel serve as the primary ally throughout most of the Halo games. They also appear in Halo: Reach, but the UNSC Army is the main NPC ally. Halo 5: Guardians is the first game in the main series not to feature them at all in the campaign.
Halo: Combat Evolved
In Halo: Combat Evolved, almost all Marines carry MA5B assault rifles, though occasionally they can be seen with a Sniper Rifle (as seen in the level 'Assault on the Control Room' and 'Halo') or the Shotgun (as seen in the level '343 Guilty Spark'), and very rarely the Covenant Needler or Plasma Rifle (as seen in the levels 'Assault on the Control Room' and 'Truth and Reconciliation') when no UNSC armaments are available.
They are relatively accurate with these weapons, with a hit percentage upwards of 50%, 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 (mainly when Ghosts get near or on a group of elites) in combat. They don't seem to be able to use rocket launchers even though there are often rocket launchers around the bodies of their dead.
Marines will strafe and avoid fire, but will not usually seek cover. Enemy fire will also render them unable to shoot or move, which makes them nearly useless on higher difficulties. 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 anything in their path.
Marines are conscious of fratricide. 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 kill you.
Marines hitch rides on player-driven Warthogs and Scorpion Tanks but are only capable of piloting Type-32 Ghost by themselves, but they are not very skilled doing so, and will often run you over by accident if you are not careful. They will remain on the Warthog if the player leaves the driver seat (although will exit both Scorpion and Warthog on the level Assault on the Control Room if left alone). 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. They are only seen driving by themselves in the cut scene of the Pillar of Autumn and cannot do so during gameplay.
Interestingly, the enemy the Marines are most vulnerable to in Halo: Combat Evolved are the Pod infectors 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!" or "Who wants a piece of me?!") 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) and running-in tendency makes them very vulnerable in the close-range combat, and often end up getting killed after going "berserk". If a "berserk" Marine survives a fight, he will calm down and simply return to normal.
Marines can also accidentally kill other marines or the player. When a marine is about to throw a grenade but gets shot, the grenade will drop and the explosion will not only kill the thrower but also anyone near it including the player.
In Halo 2, the Marines are much more intelligent than in Halo: Combat Evolved.
They are more adept with vehicles, and are able to drive the Warthog, the Type-32 Ghost, the Wraith, and the Scorpion, albeit a bit clumsily. They can now melee sleeping Grunts and will even attempt assassination kills if they catch the enemy unaware, although they lack the brute strength to kill Elites in this manner. Also, they are now able to take cover behind walls, corners, and obstacles with their backs up against them when under fire, something the players themselves are unable to do.
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 SMG or with heavy weaponry such as the Rocket Launcher or the Fuel Rod Cannon.
Unlike Spartans and Elites they lack the new ability to dual wield. However dual magnums are sometimes found next to certain marines or additional weapons e.g. an ODST on Delta Halo with dual pistols.
Although their friendly fire incidents have decreased, contrary to popular belief, Marines do use fragmentation grenades in combat (more likely when betrayed), albeit quite rarely, and are unaware of splash damage from heavy weapons, sometimes firing a rocket into a nearby wall and killing themselves.
The Marines are far more interactive, sometimes speaking to each other while in combat, taunting their enemies and wield a greater variety of weapons. They will more adeptly drive vehicles, and now have the ability to drive the Warthog, Scorpion and Ghost, and are now able to flip the Ghost. The player can also switch weapons with the Marines to make sure that they are well armed or that the player has a good team with him. 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.
Generally, taking out two marines separately in succession will instantly make them betray the player, unless they are taken out at the same time using an explosive weapon. 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 sometimes 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, or sometimes verbally forgive the player.
Changes from Halo: CE to Halo 2
In Halo 3 they can, once again, utilize the Grunts' weakness for naps by using stealth and hitting them, usually with a kick, but they can also hit something with their rifle. However due to the brute strength of the many races of the Covenant, UNSC Marines will usually lose in melee combat with vicious ballistic creatures such as the Brute, meaning they often have to rely on their group-tactics to defeat one. They seek cover much more often and even put their back up to the wall. They will 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 occasionally will make mistakes, such as running off a cliff while driving a vehicle. Marines/ODSTs seem to be able to react with the environment around them. For example, if a Ghost is about to explode, they will stand back or walk around it and continue after it explodes. Despite this, their AI was considered poor compared to the enemy AI (such as the Covenant and Flood).
The player can still exchange weapons and ammo with them, though they won't accept Brute Shots, Gravity Hammers, Sentinel beams or Energy Swords (Except Johnson on the level Halo). Marines in Halo 3 have infinite ammo for their weapons, which can be exploited by giving Marines heavy weapons.
After killing two of them, any that are left in the area will come to kill you. 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. Their appearance has also changed, and they have a lot more face models.
Marines are as vulnerable to the Pod infectors as they were in Halo: CE. If a Pod infector attaches itself on a Marine, they will grapple for a second or so, to give the player a chance to shoot it off, but if the player does not, the infector will mutate the Marine into a Combat Form. However, it is possible to "short circuit" the mutation by firing on the new Flood host, and cause the human to not fully mutate into a combat form. It is also possible to shoot off the Pod infector, but the Marine will likely take some fire in the process. Marines are extremely ineffective against the Flood if not given Shotguns or Assault Rifles, and like Halo:Combat Evolved, will take extremely heavy casualties fighting them.
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.
Marines have also gained the ability to flip vehicles, which compensates well for their somewhat poor driving. They can flip the Mongoose or Ghost by themselves, and can flip the Warthog when in groups of at least three, and strangely, they can flip the 66-ton Scorpion in groups of only five.
Marines' accuracy is also much greater and it seems they now have the ability to lead their targets, allowing them to take down Brutes driving Choppers or Grunts driving Ghosts with greater ease. This can be seen if the player drives a Mongoose with a Marine passenger using a Rocket Launcher on The Ark.
Changes from Halo 2 to Halo 3
Halo 3: ODST
For the most part, Marine allies in Halo 3: ODST are near-identical to their counterparts in Halo 3, though there have been a number of improvements made by Bungie. For one thing, driving and movement AI is far superior, including their pathfinding ability. Accuracy has also been increased, whether on foot with handheld weapons, or using turrets. "Gunners" using the machinegun turret of an M808B Scorpion tank are also improved, with better targeting than their past incarnations.
In Halo Wars, Marines are built in "squads" of four (or five when you upgraded them with New Blood).
Their starting special ability is a grenade attack which can be upgraded to RPGs. They have up to three upgrades:
In addition, if the player selected Captain Cutter as their Hero, the Marines can be upgraded to ODSTs, which have more maximum health, deal more damage, and have higher defense. In addition, ODSTs can be deployed anywhere on the battlefield with the "ODST Drop" Hero Ability.
Marines armed with the M6634 flamethrower are built as separate units called "Flamethrowers". Flamethrowers received upgrades such as Flashbang grenade, which stuns enemy infantry temporarily, and Napalm Adherent, which causes enemies attacked by Flamethrowers to burn for several seconds after the initial attack, causing damage over time. Like Marines, Flamethrowers can be created at the UNSC Barracks or trained via a deployed UNSC Elephant, though like the ODST upgrade, the latter is only possible if Captain Cutter is the selected Hero.
The marines' intelligence appears to have degraded significantly with several notable flaws. Marines appear to fire at recently deceased enemies for rather prolonged periods of time, even in the middle of combat, which can get them killed.
Marines tend to have an extremely poor performance in vehicular combat. While driving, marines will often drive rather slowly and not take any form of evasive maneuvers, which can easily spell doom to players on higher difficulties. While their performance on a turret is much better than behind the wheel, they suffer the same flaw of shooting dead enemies while ignoring live ones. It also does not help that marines have a maximum range that is smaller than that of enemy vehicles, meaning that they will not fire upon an attacking ghost until you're practically right next to it.
Finally, while marines are generally accurate and effective with heavy weaponry, they have been known to accidentally kill themselves or their allies, the player included, in close-quarters scenarios.
Halo 5: Guardians
Marines are not present in Campaign mode, though Liang-Dortmund private security forces take essentially the same role in missions set on Meridian. They will generally follow the player. They are equipped with Sniper Rifles, M20 SMGs, MA5D assault rifles, BR85 battle rifles, and M6H2 magnums. They have reasonable accuracy. If shot or hit with a melee attack from the player, they will yell something at you. No matter how many are betrayed, the survivors will not attack you.
For the first time in the series' multiplayer (excluding the Halo Wars strategy games), Marines make an appearance in multiplayer, serving as NPCs in the Warzone gamemode. Here, they defend captured bases. They spawn immediately after capturing a base and will not move outside of it. Unlike in previous appearances, the armour of the Marines is not camouflage, but instead colored in the color of the team they spawn for (Red or Blue). When approaching a friendly base, the Marines will wave to the player and speak to them.
They will not trade weapons and do not pick up weapons, nor will they enter vehicles. They are equipped with an MA5D Assault Rifle or a BR85 and frag grenades.
In terms of quality, the Marines can hit very accurately and constantly fire when enemies are within range. Three Marines can easily over power a single SPARTAN. However, they can be easily killed by headshots or silenced weapons. Like every other Warzone AI enemy, Marines have their health increased a bit.
Halo Wars 2
Marines come out of Firebases with a squad of four by default and cannot throw grenades. The number of Marines per squad can be increased to five if player researches the Combat Technician upgrade. They have two upgrades for combat and if paired with a Mastodon, they can be very lethal all-rounder units.
Changes from Halo Wars to Halo Wars 2