The Great War is the term used to refer to the global thermonuclear war that occurred 21st March 1998, and the culmination of the decades-long arms race between the United States/NATO and the Soviet Union/Comintern Combine.
The Great War was triggered after the Citadel Starstation broadcast a distress signal reporting an incoming Soviet nuclear attack, followed by the destruction of all satellites present in orbit. In panic, all nuclear powers sent 90% of their arsenals skyward, wiping out human civilization in the span of just 13 minutes. The once lush blue planet was turned into a wasteland wracked by radiation storms, rampant mutations, and extreme violence.
It was not an accident, but a deliberate move by the Cochise AI, which considered the eradication of humanity the only way to preserve itself in the long run. The Citadel distress signal was faked by the AI, a trivial matter, as the Cochise was instrumental in accelerating the Starstation's construction.