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.
The players find themselves in the Greater Seattle area; formerly a large town, the majority of buildings sunk into the sea after the Great War. The two nearby volcanoes are now semi-active; and occasionaly throw great mud drifts across the hillsides. Seattle is not controlled by one power, and is largely inhabited by groups of Slavers and Bandits, with the most powerful group, the Khans, making their home in and around Vault 52, within the city. The New California Republic is the closest thing the wasteland has to civilization, and frequent patrols of NCR rangers are often seen having firefights with the various denizens of the city. Seattle would be a great asset for the Republic, and a great deal of blood has been shed over its attempted capture.
The year is 2282. The players have lived comfortably within Vault 12 their entire lives. The Vault is home to around 120 people of all ages and roles. The front door to the Vault has never been opened in the 104 years since the war. Like most other Vaults, Vault 12 is completely sustainable, and houses a G.E.C.K, or Garden of Eden Creation Kit, allowing the inhabitants to maintain a limitless supply of fruit, vegetables, and pure water.