Yokohama Base was the location for two different military bases staffed by two different armies. Prior to the BETA Invasion of Japan, Yokohama Base was a military complex staffed by the Imperial Army, and was also the home base for Unit 90, an anti-BETA bioweapons research group. The base was lost to the BETA after the invasion, and became the site of Objective 22, the Yokohama Hive.
In 1999, Objective 22 was captured by allied forces in Operation Lucifer, along with its Hive Reactor intact. After its reconstruction, Yokohama became the main base of the UN 11th Force, and the headquarters of Alternative IV, where its chief researcher, Professor Kouzuki Yuuko, is stationed at. There are many reasons why the base was built on top of the Yokohama Hive; it was to serve as both a research facility and a means of safekeeping the newly-captured Hive Reactor, and the site was also ground zero for the first two G-Bombs to be used in human history; the base itself served as a sort of experiment to see what were the long-term effects on humans living on a G-Bomb site.
The base is well-equipped, with shuttle launch sites, two large-scale runways, two mass drivers, vast training grounds, and is of close proximity to Tokyo Bay; static defensive emplacements are located around the facilities, and the base's various large access gates can be sealed to prevent entry by enemies, although unsealing them again would take at least half a year. Underground facilities are vast, owing to the fact that the interior of the base was derived from the original interior construction of the Yokohama Hive. A main shaft connects the lowest levels to the topmost floor, and is protected by five barrier gates.
The base's main area is the central gathering area, a wide open space in the interior of the base that is located one level below the base's surface; specifically, where the training field is. it is from this level that access to the other underground facilities of the base is allowed. The Yokohama Base's underground hangars are large enough to allow storage of several XG-70 units.
Located at the deepest level is the Yokohama Hive Reactor, accessible either by the access hatch at the bottom of the main shaft, or the control and monitoring room at Level B33, which will give one a clear view of the Reactor. The Reactor not only serves as the base's primary object of research, but also as an energy source for the base.
- 2nd Lieutenant Kagami Sumika
- Yashiro Kasumi
- Sergeant Jinguuji Marimo
- Lieutenant Irina Pyatkh
- Master Sergeant Kyouzuka Shizue
- Homura Manami
MP personnel under the command of the UN 11th Force. During normal duty they police the interior of the base and prevent unauthorized personnel from entering, and are also deployed as security forces within the base in the event of an external attack.
Troopers armed with Type-87 Feedback Protector systems modified to hold weapons. They serve a variety of roles, ranging from security to base defence. During the Defence of Yokohama Base, four companies of troopers were lost.
- 101st Mechanized Infantry Regiment
- 108th Mechanized Infantry Regiment
- 116th Mechanized Infantry Regiment
The 11th Force uses troops borrowed from both the Empire of Japan and its own UN forces to form the seven main TSF battalions that are housed in Yokohama Base, in addition to several battle tank squads and artillery/MLRS batteries. Most of them were annihilated in the Defence of Yokohama Base.
- 1st to 7th Tactical Armored Battalions
- Hawk, Dragon, Joker units and Blizzard Squadron used F-15Js, while other units were equipped with F-4Js.
- They have been heard using the following callsigns: Hornet, Sword, Cracker, Storm, Axe, Sobat, Hound, Shark, Falcon, Tiger, Victor, Tomahawk, Hermit, Lotus (Flight), Gambler (Flight), Blizzard (Squadron). The following callsigns are those used only by the 7th Tactical Armored Battalion: Rapier, Grizzly, Circus, Dragon, Joker, Flame, Liger, and Hawk.
- Main Battle Tank forces
- The tank forces of the 11th Force use Leopard 2 MBTs.
- Artillery forces
Yokohama Base maintains a few Air Support Groups within its complex, which are tasked with providing combat firepower support in the event of an attack.
- 1st to 7th Air Support Groups
- Most of the air support groups were scrambled during the Defence of Yokohama Base, and were later diverted to surrounding UN bases; included amongst them are the 105th Air Support Squad, a helicopter close-air-support squad. Most if not all of them use the AH-64D Apache Longbow as their primary aircraft.
- ~1999: Plans for the (re)construction of Yokohama Base are first laid down.
- ~2001: Yokohama Base reaches 70% operational capacity.
- October 22nd, 2001: Shirogane Takeru arrives at Yokohama Base.
- November 29th, 2001: The TSF_Components#XM3 TSF operating system, making use of new operating concepts, developed by Kouzuki Yuuko with the aid of Takeru, begins demonstration tests at Yokohama Base in absolute secrecy.
- December 5th to 6th: Yokohama Base deploys the 207th Tactical Armored Platoon during the 12/5 Incident.
- December 10th: The XM3 Trials begin to test the XM3 OS for mass-production, and is followed by the Yokohama Base Incident.
- December 10th to 17th, 2001: The 00 Unit is completed.
- December 29th to 30th, 2001: The Defence of Yokohama Base; surviving BETA forces from Operation 21st attack the base. The results of this almost causes the destruction of Yokohama base along with the 00 Unit and incomplete XG-70b and XG-70d. This is narrowly averted at the cost of over 10,000 killed, wounded, or MIA personnel and the effective annihilation of all military forces stationed there.