It was me while playing around with my nun chucks. I accidentally hit the switch. Sorry.

Who "fired first" is a trivial technicality. Cynicism, greed, jingoism, and rampant consumption on both sides destroyed the Pre-War Fallout world.

It has happened in the real world. Well, not with nukes, obviously. USSR troops infiltrated to Finland's side and shelled a village in Russia. As a result USSR was forced to start a war with Finland in self defence
The incident is know as https://en.wikipedia.org/wiki/Shelling_of_Mainila.

Detonating a nuke on your own soil would give you fighting chance to wipe out your enemy completely with the "counter attack" before they have time to react.

I have pretty much zero interest in the Old World. The Old World is dead, and the only parts of it that interest me are the remnants of it that still echo its heartbeat (The Sierra Madre Casino, Big MT, etc).

I have absolutely no interest in finding out who started the war, because it essentially means nothing to the world of Fallout. It doesn't impact the story in any significant way. It doesn't change the result or impact of the nukes, and finding out wouldn't mean anything to the people currently alive. You might as well be asking if the guy who designed what became John Henry Eden was left or right handed.

I agree, and chalk it up to a horrid misunderstanding.

The aliens want the guy to talk, and since the only information he has that's worth anything are the codes, he doesn't want to talk.

The aliens, of course, want him to fill out his specimen audio log thing. Since he suddenly clams up and is presumably resistant to the zappers... out come the interrogation tools.

Weren't there PA troops in China fighting through hordes of chinese infantry at the time the bombs fell?

I could see China being the one that did it. But then everyone in the Falloutverse is some horrible dystopia even before the bomb. I wouldn't put it past the police state of the US firing first, using tactical nukes in support of the PA troops with no understanding of the consequences. Maybe the rump USSR thought that Vladivostok was going to be the next front line, and launched attacks on US positions near the border, triggering the exchange. I don't think we'll ever find out, the old world is long dead by the time the sole survivor awakens.

Both President Richardson, and terminal entries at Black Mountain in NV, suggests it was china who shot first.

Neither had any particular reason to lie, so I don't see any real reason to doubt them.

I wouldn't say Richardson would have motive to lie, but I do think his predecessors would have reason to lie to him. The Enclave thought of itself as a continuation of the legitimate government of the United States of America, so I doubt that they'd be laughing about what happened and going "All that devastation and the ruined world? We started it all, baby!" They'd likely exercise historical revisionism to make themselves seem just as much the victims as everyone else, regardless of whether or not they fired first. Especially since there's no one who can really challenge their version of the story.

That said, the terminals of a scientific research institute would have little reason to lie, though we know that Dr. Klein did lie about research at least once. I think those entries are more trustworthy than the president of the Enclave, even if they're both giving the same information.

It is just as much of a mystery, as the Shot Heard Around the World.

Pointless as far as fallout 4 goes.... Or any of the old fallout games. However this can become a major plot point in fallout 5-6 As that will most likely deal with the deeper secrets of vault-tec. But that is just random speculation.

Personally I like the idea that it was a false alarm that resulted in the nuking of the world. Its just the sort of messed up missunderstanding that just seems perfect for Fallout.