Foreign invaders are seen from Takashi Shimamura's high school during the Sustainable War.
The Sustainable War is a concept and event in Ghost in the Shell: SAC_2045. It was launched in 2042 by the American Empire under AI Code 1A84 in cooperation with the other members of the G4 (the Russian Federation, China, and the European Union), and officially classified war as an industry to boost the global economy. In effect, the G4 nations went to war with each other in a "sustainable" manner managed by AIs: fighting deliberately avoided damaging major cities and essential infrastructure, mercenaries, and autonomous war machines were heavily used, and the goal of most battles was to destroy enemy structures, vehicles, and equipment while taking reasonable measures to capture rather than fatally harm human combatants when possible. In other words, the conflict would be sustainable as almost all losses could, in theory, be perpetually replaced by the military-industrial complex. This idealized vision of warfare nonetheless displaced populations and turned smaller cities and towns, such as Palm Springs, California, into battlefields.
Following the Synchronized Global Default in 2044, the Sustainable War became the only way for the G4 nations to keep their economies functional. The Default also created countless destitute raiders, who risked their lives amid the Sustainable War to eke out a living or wreak vengeance against those they blamed for their poverty, the "one-percenter" elites who continued enriching themselves through the war.
In 2045, the mercenary unit GHOST was hired to guard a city in California from raider attacks. GHOST was formed by Motoko Kusanagi and other members of the former Section 9 as a way to continue using their military skills to make a living through the Sustainable War after Section 9 was disbanded.