Here's an alt civil war scenario that I like, the British help the CSA for whatever reason you can come up with including manpower and weapons, BUT...
This serves to infuriate the Northern populace due to, among other things possibly, the prolongation of the war and much more casualties. Eventually the US wins having just as large an army as they do IRL after beating the CSA. In retaliation they take enough of Canada with that huge army that the British eventually give up the rest.
Meanwhile during all of the Civil war fighting, some Canadians resent fighting at all in what they see as none of their affair while some others resent being on the side of the slave-holding CSA and refuse to fight in a war they oppose. This, on top of no or little British support due to them being stretched to the limit due to all of the global demands on their manpower, creates less military opposition when the US does eventually invade.
Post civil war this creates a huge gap between the British and the US. I wonder on what side the US would fall when WWI and II happens in this scenario.
As a possible POD, maybe the US does at least a little better in the war of 1812, not winning any territory in the end, but prolonging the war and forcing the British to commit resources they desperately need elsewhere. This causes the the Brits to lose one or more battles they should have won. This pisses off the British quite badly for quite awhile afterward, because many of them blame the US for their losses.
In between the war of 1812 and the Civil war the tensions mount between both sides because of the British impressing US citizens and ship/sailors as well as raiding of a US port/settlement or shipping convoy under False or Pirate colors with the US finding out anyway. Then along comes the Civil war and viola.
I know it's full of holes but, whatever I'm tired and don't care.