Why does the US have to annex Canada? Personally, I always disagreed with that in the books. Most of Canada's geography is uninhabitable. Nowadays in our timeline, most of Canada lives near our border, and I assume they did back then. The change I always had in mind was for the US to create not one, but two allied states. Quebec, and Republic of Canada (after a few years of US occupation). Canada would be a backup ally during WW2 or at the very least neutral but leaning towards the US.
I don't really see why the US wants to annex a country that is barely habitated above the US-Canadian border. I could see them annexing New Brunswick, Nova Scotia, Prince Edward Island, and the piece of Ontario that is in the Great Lakes though.
Quite simply, Canada is a lightly populated country where 99% of everything of importance is within 100 miles of the extremely long border. It's easy to control everything important, and you don't want a big enemy military base with a small country attached sitting right above your main industrial region. So annex Canada, swamp the region with settlers, and set up some naval patrols to remind London who's boss in the hemisphere. Insurance policy, basically.
And another non-sense annexation is CSA at end of GW2. How USA even think that they could annex country which has been independent more than 80 years with millions of people who are not going to accept that? Or if they would think that, it is not going to be easy thing and probably failure at end.
No American government would be able to let the CS go as a coherent independent nation after the CS nuked an American city. Look at what we did to Japan IOTL, WITHOUT ever launching a coherent ground invasion. We damn near completely disarmed the place and caused a sea change in its politics that are still reverberating. It is our most reliable ally in the region except maybe Taiwan and has only elected a leader from a party other than the main pro-American party ONCE (and his administration fell apart soon after) since we occupied the place. And that was a country all the way across the Pacific, composed effectively entirely of non-white people, WITHOUT a US city being a radioactive crater.
(this is handwaving all discussion of the CSA's military performance in TL-191 canon, which as has been litigated an infuriating number of times, makes no logical sense and basically depends upon a series of handwaves making the CS not only a powerful industrialized nation but also a powerful industrialized nation with its primary opponent sabotaging itself repeatedly for no reason)
I feel like you could write a whole novel about Cassius and the Red Rebellions during the First Great War. That being said historically agrarian societies have found Anarchism rather than Marxism more appealing. I feel like labourers on the plantations where the rebellions break out ITTL would be more likely to organise along anarchist lines.
Considering the more leftward swing of the US, I think it's beyond reasonable to have the US back the Red Rebellions. From a how I would write it POV, have a reporter with a radio get embedded with the rebels somehow, then have them regularly send back reports about the Noble Negro Rebel Heroes fighting bravely for Father Abe and Comrade Marx. They go from raiding plantations to prepping for a big strike on a CS military base that they take over and use to launch a coherent breakout attempt towards advancing Union forces.
Call it "Three Hundred Thousand More" after this song: