Although the Fallout universe and the real world share a common timeline until about 1945, the defining moment of the Fallout universe was the Great War of 2077. On October 23, 2077, the United States, China, and other nuclear-armed countries commenced a brief but rapid exchange of nuclear strikes. Although the conflict lasted only two hours, the destruction it brought was staggering and complete: more energy was released in the early moments of the war than in all previous global conflicts combined.
The world that remained was harsh and unforgiving. 90 years after the Great War, humanity survived but struggled to do so. Above ground, small bands fought over the crumbling remains of once great cities while mutants prowled the irradiated wastes. Underground, a fortunate few enjoyed the relative safety and comfort of the Vaults. Some fought to keep the world out, others to connect with and re-populate it. Wherever humanity survived it was under constant threat by psychotic mutants, rogue machines, vicious raiders, and all manner of hostile mutant creatures.
Most of the events of the series took place in the United States. The first two games - Fallout, Fallout 2 - were set in the Core Region: the remains of the West Coast. The Master and the Enclave are the region's main threats. The third game, Fallout Tactics, took place in the Midwest- between the city of Chicago and Cheyenne Mountain in Colorado. The fourth game, Fallout: Brotherhood of Steel, took place in northern Texas in the town of Carbon and the ghoul city of Los. The fifth game, Fallout 3, was set on the East Coast of the United States in an area called the Capital Wasteland; this included the remains of Washington D.C., parts of Virginia, Pittsburgh and Maryland. The sixth game, Fallout: New Vegas, took place in the Mojave Wasteland which included parts of California, Utah, Arizona, and Nevada.