Almost all of Fallout is focused on the post-apocalyptic. The franchise shows what the United States of America could look like after a nuclear fallout, but fans have always been invested in what happened before the bombs dropped.
One question lingered on the minds of many: who dropped the first nuke? Who started it all? Well, now Fallout's co-creator Tim Cain has our answer. Speaking with YouTuber TKs-Mantis, Cain revealed that the reason the US got nuked is "bio-weapons were illegal and somehow China found out we were doing FEV [Forced Evolutionary Virus]"
While Cain doesn't explicitly state that it's China that started it all, this seems pretty concrete. The FEV was a bio-weapon created by the US sometime in the 2070s in order to create super soldiers. After China discovered what the US was doing, it decided to pull the biggest trigger of them all.
As Cain remained somewhat elusive in his answer, it seems the mystery can go on, but for now at least this appears to be pretty solid proof that China shot first, leading to the destruction of the world, but it only did so after the US developed bio-weapons.