[Rivet City, a sea carrier located southeast of D.C]
After world war one (1913-1918 )the United States tried to devise fusion power resulting in the whole country becoming hegemonic and having less reliance on petroleum. However, this is not achieved until 2077 shortly after an oil drilling conflict off the Pacific Coast pits the United States against China. In 2077 It ends with a nuclear exchange resulting in the post-apocalyptic world in which history takes place. It is emphasized after 60 years later that nobody knew who sent the first missile.
So the year of 2077 is where the story of Fallout begins, forged by what they called "The Great War".
Before the nuclear exchange took place, great underground Vaults were constructed across America, supposedly to protect the populace from the dangers of radiation. Although only 122 known vaults were constructed, over 400,000 would be needed to protect the entire nation. This is because the Vaults were not intended to save humanity; rather, they were social experiments being conducted by the United States government. Most vaults featured some variable to test how certain things influence people (and presumably the personal characteristics of the vault's occupants) such as Vault 69, which reportedly contained 999 women and one man.
Each installment of the series takes these facts as the context to the subsequent adventures: much of the landscape the people travels through is scarred with wreckage as well as radiation. These effects are not limited to the environment. Mutated survivors - those who lived through the attack outside a vault - are often physically unrecognizable as human. Even livestock - mostly represented by cows - are rarely if ever seen with less than two heads.
Now after the reopening of the vaults, humanity fights for survival, kill or be killed some say. and why? Well..
War.. War never changes..