Japanese Strategic Self Defense Force
The Japanese Strategic Self Defense Force (JSSDF) is a military organization, the main branch of the Japanese military under the direct command of the Japanese Ministry of Defense. It has a background role in the Evangelion TV series, serving in a military support role for Nerv. The JSSDF serves as a primary antagonist in The End of Evangelion.
The JSSDF was formed in 2003 in response to the outbreak of war between China and Vietnam over the Nansha Islands as a result of post-Second Impact tensions, which prompted the reorganization of Japan's military. As a result of this new political climate, Article 9 of the Japanese constitution (which forbid Japan from maintaining an offensive military force after World War II) was apparently abolished.
The JSSDF functions in coordination with United Nations forces in providing support for Nerv and the Evangelions when they attempt to repel Angel attacks. United Nations forces in 2015 are often heavily composed of the armed forces of local militaries, which have all been united under the aegis of the UN. Thus frequently when "United Nations" forces are supporting Nerv in combating the Angels, they are in fact heavily composed of JSSDF units, commanded by their own generals, but backed by the authority of the UN.
The "JSSDF" is composed of both land and air units, and presumably naval forces. There are sub-branches of the Strategic Self Defense Force which focus on each military subdivision, for example the "Japanese Air Self Defense Force" (JASDF) is the sub-branch of the JSSDF which is composed entirely of air units (though the JSSDF proper does field its own VTOL airships, etc.)
As the military of the entire nation of Japan, the JSSDF has access to extensive weaponry and vehicles, both foreign and domestic. Unlike Nerv, the JSSDF is overwhelmingly a conventional military force. They are also heavily involved in the research and development of several directed energy weapon projects, such as the positron rifle.
The pinnacle of the JSSDF arsenal are N² Mines: the most powerful explosive devices ever created by Man.
The SSDF's mechanized ground forces operate conventional vehicles like main battle tanks, Humvees, mobile rocket and artillery MLRS platforms and other mainstays of modern land warfare.
The JSSDF's air wing operates many large VTOL gunships, which effectively wield the firepower of a flying battleship. Its arsenal also includes fixed-wing bombers, and large "flying wing" craft used to transport Evangelions (be it shipping an Eva between North America and Japan, or deploying Evas directly into combat against Angels).
Infantry forces wear black bodysuits and helmets, while officers and rearguard staff wear berets. Troops appear to wear little or no body armor, as their suits cannot stop 9mm handgun rounds, apparently sacrificing safety of individual soldiers in exchange for rapid deployment and manueverability. Frontline personnel carry riot shields when assaulting fortified positions.
The standard-issue small arms are the Heckler & Koch G11 assault rifle and the Fabrique Nationale P90 submachine gun and Hi-Power pistol. Select squads are also armed with rocket-propelled grenade (RPG) launchers or flamethrowers.
Role in Neon Genesis Evangelion
After the forces of the JSSDF (functioning under the command of the United Nations) are unable to defeat the Angel Sachiel in Episode 01, even with an N² mine, command of the overall global battle between mankind and the Angels is transfered to Nerv. Following this, the JSSDF supplies conventional military support for Nerv throughout the series.
In Episode 06, Nerv requisitioned the JSSDF's prototype positron rifle from their R&D divsion, for use against Ramiel. In Episode 07, Misato Katsuragi asked Ritsuko Akagi if the SSDF was behind the development of the Jet Alone robot project, but Ritsuko said that it was developed by a private contractor. In Episode 18, In Vice-Commander Kozo Fuyutsuki specifically ordered that the Eva-03 activation accident be dealt with before the SSDF could intervene.