Here's where I'm confused with the divergence. I thought the Great War was caused in part by oil getting used up. But if the society is atomic, would that matter?
A couple things:
First, not all of society was atomic, or could afford to be. Pre-war US was a place of extremes: it seemed nice if you could afford it, but the robotics revolution was simultaneously driving a lot of people out of work. (See: The Pitt and the Union-busting robots.) Even for the US, oil was important. A lot of the tensions in the pre-war US were as much social as anything else.
Two, not all the world was as atomic as the US. The US was a world-leader in atomic energy, by most accounts- possibly an extension of the insularity that defines the Fallout-verse pre-war US. But there was already a global energy crisis, even as the US was putting nuclear engines in cars, and Europe was fighting in the Middle East for oil, so it's pretty safe to assume that most places in the world were even worse off, less atomic, and more dependent on the dwindling oil.
The Big MT and FNV really hits that the pre-war US was in sight of solutions to the resource/energy crisis, but not there yet. The Sierra Madre vendors were basically matter repurposers that just needed energy and raw materials to produce finished goods. Nuclear energy could produce the power, but there's solar as well. Even Liberty Prime was just one technical breakthrough away from being a super-powered giant robot. Things were jusst that close.
What really killed the world was that everyone was more determined to not be the only ones to lose. Which is understandable, considering how bad the 'winners' were, but the world was just a few weeks (days, or even hours in the case of Las Vegas) from being far better off and able to recover than it was. But no one wanted to let anyone else 'win' or 'get away with it': European forces left the Middle East when there was nothing left to fight over, but nukes followed back. The US ejected China from Alaska... and then went on to invade Beijing because Reasons, and China wouldn't go down alone. The world turned on itself, everyone nuking everyone else so that there was no one left unnuked who could be the last person standing.
I think in this continuity Canada actually got annexed? I recall power armor troops got sent to Toronto to put down protesters.
Yes it was, the US wanted a more united force to fight the Chinese. There was some guerrilla resistance, but I don't think it amounted to anything.
There's a newspaper building in FO3 in which you can read some articles (pro-US propoganda), and it was referenced in the trailer of... FO2? The Survivalist in Zion also served there when he was in the military.
Basically, the US was using Canada as a land bridge to Alaska and cutting down large amounts of forests to support the war effort. Canada was willing at first, in the 'I'll give you permission since you'd just do it anyway' sort of way, until something changed and the US invaded to remove the possibility of refusal. Most distinguishing thing I can think of is that power armor was used in the occupation, and I think(?) there were fliers for Americans to try and settle in Canada?
I've mused a few times about a Canada-based Fallout. Wouldn't be that different from south of the Border (which is to say- the nation is the backstory, not the current identity), but I did think it'd be amusing to have two Vaults still fighting the war between themselves. One Vault being a Vault overran by Canadian resistance fighters before the bomb fell, leading to a hypernationalist uber-Canuck vault after the Canadian resistance overwhelmed and ruled over the American civilians inside. The other Vault, hypernationalist uber-American, would turn out to be a Vault where the experiment was on if Canadian civilians could be indoctrinated into Americans. So you'd have a vault of former Canadians who think of themselves as Americans, and a vault of former Americans who think of themselves as Canadian, and all the outsiders and local tribals look at them and wonder why anyone gives a ****, if anyone even knows what 'America' and 'Canada' even were.