USA and China were fighting a war started by Chinese invasion of Alaska in 2066.
USA drove China out in 2077 and invaded China itself.
USA reached Yangtze river and conquered many important cities such as Nanking and Shanghai.
Chinese supply lines collapsed.
China launched first strike in October 23rd 2077.
USA could barely respond, and the Enclave's president (and thus US President) in Fallout 2 notes that "We barely got our birds up [...] but at least we kicked those damn reds back into the stone age".
All nuclear powers launched and no country was spared.
US government retreated to an Oil Rig in Pacific Ocean, leaving the country on its own. US army shattered and the Brotherhood of Steel along with the Enclave's military are the only remnants of the pre-war army.
We do not know what happened to world post-war, but considering that there are several foreigners appearing in Fallout games it means that the whole world is in the same shape (or worse) United States is. There are at least two known immigrants from Britain, Moriarty and Tenpenny. Also Dukov who is Russian. I recall there was also a Canadian in Fallout 2 and there is Raul from Mexico.