World War IV, also known as Non-Nuclear World War IV and the Second Vietnam War, was a world war in the Ghost in the Shell universe that occurred sometime between 2015 at the earliest and 2024 at the latest. It is described as a "non-nuclear" conflict because the countries involved did not utilize nuclear weapons, which was because they had already expended their nuclear weapon stockpiles in the course of World War III.
Although referred to as a world war, the term "World War IV" appears to encompass multiple regional conflicts that happened to break out in the same period and for similar reasons, primarily the devastation and dysfunction brought on by World War III. At least four major conflicts are described: a war centered on Southeast Asia and China that gave World War IV its alternate name of the "Second Vietnam War" and contributed to the Refugee Crisis in Japan years later; a war of attrition between Europe and Asia that became deadlocked in the summer of 2020; an invasion of Mexico and South America by the American Empire backed by United Nations peacekeepers in 2020 that eventually stalled out due to endless guerilla attacks; and the Peninsular War or Second Korean War of 2024 that ended with the reunification of Korea under a democratic government.
When the dust settled, the American Empire and mostly-neutral Japan emerged as the foremost of the world's countries, with the former boasting its military strength and the latter its resurgent economy. Elsewhere, the number of independent states and sovereign regions multiplied, and hundreds of mercenary groups emerged, populated by remnants of armies once connected to nations that no longer existed. In the aftermath of World War IV, the world remained more divided than ever, and there were many locations where sovereignty fell into question and no one was really sure who owned or governed what.