Alpha Base (Installation 04)

Alpha Base was the main tactical Firebase used by the remaining UNSC forces after planet fall on Installation 04. It was located on a small mesa several kilometres down-spin from the crash site of the Pillar of Autumn. The mesa itself housed a collection of Forerunner structures as well as a system of caves stretching beneath the facility and linking with the larger subterranean system of the ring. Wellsley tagged the base as HS2604.

The humans set up camp around the base using the supplies they had scrounged from the Autumn and their Human Entry Vehicles. They lived in cubicles and ate mostly Meals Ready to Eat. A good deal of the activities also took place in caverns underneath the facility in the Forerunner structures. It was abandoned by the humans shortly before the destruction of the ring, due to their taking of the Truth and Reconciliation and the growing Flood presence in the area.

History
The Covenant occupied this mesa when they discovered the ring, using mostly a light compliment of Grunts, Jackals, and Elites, as well as several Banshees to guard it. It provided a good view of the surrounding areas, and the steep embankments around it made it hard to reach from the ground. Strategically placed Shade turrets made the mesa hard to assault from the air as well.

When the Pillar of Autumn was attacked above the Halo by opposing Covenant forces, the ODSTs, under command of Major Antonio Silva, abandoned the ship aboard Human Entry Vehicles, and most of them landed around the base. Silva saw the tactical advantage to the butte, and ordered First Lieutenant Melissa McKay to take it. While some of the ODSTs were able to attack the butte with supporting air support, some were lead into what was known as a back door. After elimination of all immediate threats, this team entered the back door and took a lift to regroup with the rest of the squad. It is unknown how much of a defense Silva put into guarding this pass, but apparently the Covenant were unable to use this backdoor to gain access to the base.

Despite now having this strategic foothold, Silva still had a great deal of problems. The ODSTs were equipped with enough food and supplies to last 2 weeks. However, that was not the case with the 63 crewmen that the Master Chief was able to rescue. Also, the dropships were running low on fuel and the base was not adequately prepared for a massive counter-attack. Therefore, Silva ordered McKay to go to the downed Autumn and recover whatever supplies that she could. The raid was a success and only a minimum of UNSC casualties occurred, while several Ghosts and a Spirit dropship were destroyed. The Marines and ODSTs were able to secure a sufficient amount of fuel, food, and munitions. Also, two 50mm turrets were recovered, a contingent of LRV Warthogs, and 4 Scorpion Tanks. Unfortunately, there was no ammo or rockets for the Pelicans, limiting their usage to transport only. Some of the supplies, LRVs, and Marines were lost in the return trip due to Covenant resistance.

It was later discovered that the Flood had been attempting to break into the base and were only impeded by a single metal grate, (Which they had already used hundreds of Plasma Weapons on), leading to an unnoticed group of caverns. The entrances to these caverns were successfully sealed, and the Flood were delayed from overtaking the base. By the time they managed to break through, the UNSC forces had abandoned the base in hopes of leaving the installation aboard the Truth and Reconciliation.

The base was destroyed along with Installation 04 when the Master Chief overloaded the Pillar of Autumn's reactor.

Defenses
The defenses of Alpha Base consisted mainly of UNSC personal that survived the crash of the Pillar of Autumn. A number of stationary 50mm autocannons were in place and linked to the base's AI, Wellsley. The base had several makeshift landing pads where dropships could be docked and refueled in relative safety. Later, this position was reinforced by a group of LRV Warthogs and at least 2 of 4 Scorpion tanks that were recovered from the downed Pillar of Autumn. Due to successful raids and the attack upon the Truth and Reconciliation, at least 4 plasma turrets, known as "Shades", were set up in defense of the base. Unfortunately, these turrets had limited effectivness against dropships. Also, 15 operational Covenant Banshees were recovered. A basic perimeter was set up and monitored through motion trackers by the AI Wellsley. Pelicans had quick strike teams in place if their was a perimeter breach that could respond in about 2 minutes. Finally, a combination of LRVs and captured Ghosts were used as patrols, effectively combining power and speed.

At its strongest point, the bases defenses were as followed:
 * Battalion of ODSTs led by Major Silva.
 * Most of the Pillar of Autumn's remaining crew that were not captured or in cryo stasis.
 * Two 50mm autoturrets (AI operated).
 * High amounts of light and heavy weaponry (both UNSC and Covenant grade).
 * 4 Shade turrets.
 * At least 2 of 4 remaining Scorpion Tanks, dug partially underground for protection.
 * A contingent of LRV Warthogs.
 * 15 operational captured Banshees.
 * Captured Ghosts (estimated at about 5-10).
 * AI Wellsley.
 * Multiple landing pads.
 * All surviving Pelicans (not equipped with rockets and low machine-gun ammo).
 * Ground based motion detectors.
 * A system of trenches surrounding the base.

First Defense of Alpha Base
The base was attacked twice while during the time the UNSC was on Installation 04. This attack was led by Major Silva and his battalion of ODSTs in an attempt to wrench the protective butte from the hands of the Covenant in order to establish a place where the UNSC personnel could regroup. Silva ordered Lt. McKay to lead an assault through a pass that hopefully led to the top of the base. As McKay and her company began their climb, the Covenant launched a preemptive strike against Silva. Wellsley informed him that the attack would consisted of an estimated 100 Ghosts. With half of his forces committed elsewhere, Silva orders companies A and D to form into what is know as an infantry square. This, and the natural terrain, gave the ODSTs a better chance of survival against the Ghost's fixed weapons. In the beginning, the Covenant fought very poorly. Several Ghost fired upon each other or collided in their haste to score a kill. Eventually, a Field Commander took charge and ordered all of the riders to ride counter-clock wise and then to attack the lowest pit in the square. Human casualties increased and the square started to break. Silva responded by reinforcing the pit, getting his rocket-jockeys to rotate their salvo's, and had his sniper team concentrate on the Field Commander. As an apparent after thought, Banshees were thrown into the fight late and were quickly eliminated by M19 launchers. Finally, with the Field Commander dead, and a 50% casualty rate, the attackers retreated, some carrying extra riders on their Ghost. From that attack, 23 were KIA, 6 were critically injured, and 10 were WIA. Finally Silva could get the Pelicans to attack the butte and give McKay some support.

Meanwhile McKay was making her way up the butte. She had already lost some of her soldiers, one to a plasma grenade, but she was making a good deal of progress. At last she made it to a back door into the structure. She fired an M19 rocket into it, which struck some sort of ammo, and cleared the room of all hostiles. She and her remaining company entered to find an elevator to the surface of the butte. As a result of the battle, the UNSC managed to gain a base of operations.

Second Defense of Alpha Base
The second battle was Zuka 'Zamamee's attempt to get into the base and kill the Master Chief. Approximately 6 Spirit dropships full of Covenant and the traitorous Pelican Charlie 217 full of cloaked commando Elites participated in the attack. The Pelican managed to touch down and release the Elites before its landing pad was doused in fuel and lit. However, Zuka 'Zamamee and several Elites had already fled and began in search of the Chief. Before being killed by the Stealth Elites led by Zamamee, a valiant crewman, Cho, gave the AI Wellsley control of the 50mm autocannons, with which he was able to eliminate 1 Spirit dropship. The remaining dropships released their troops further away from the base and attempted to assault it thus. Snipers expertly inflicted heavy casualties on the disembarking troops. Two of the recovered Scorpion Tanks had been entrenched and were able to cut down much of the onslaught, including the Field Commander. With their leader dead, lesser Elites attempted to assume command with little success. In the end, all remaining Covenant were either captured, killed, or forced to retreat. Yayap and Zamamee were able to survive by stealing one of the Banshees that the Marines had been able to salvage.

Trivia

 * Wellsley suggested it be named 'Gawilghur', referencing a fortress Sir Arthur Wellesley, Duke of Wellington, laid siege to in India. Major Silva shot down Wellsley's request and stated that most of the Marines wouldn't be able to pronounce the name right.
 * One of the said four Scorpion tanks is presumed to be the one encountered on Assault on the Control Room.