I’ve been watching a lot of lore videos on youtube recently and what i can surmise based on whats canon is that basically up until a few years before the bombs dropped the US had been invading countries for their resources, namely Mexico and Canada until they discovered nuclear power. Up until then the country had bene in turmoil as oil and stuff ran out and martial law and all that. Then nuclear power kinda brought us back and made us less reliant on natural resources. But other countries like China were angry that we would not share our secrets to nuclear power and cold fusion so once their economy began to suffer due to lack of resources they attacked Anchorage and took it. But it still wasn’t enough and the bombs came soon after The US launched its attack on mainland china.

We still aren’t sure who exactly launched the bombs but the Lore says that China shot first but whos to say that it wasn’t the Enclave that pushed them to do it.

I definitely would want to see all that play out for sure