Bear in mind that the US never behaved like other nations. After World War II, we had a ton of Pacific islands that weren't ours, but rather than adopting them as our own, we gave them to the UN to decide what to do with..
Well when we are talking about annexing Canada and Mexico historical US policies become a bit less relevant.
Plus I tend to be a bit more cynical about such things.
But anyway if you intend to have some basic amount of roleplaying though your route does have the appeal of being less insane (not that its hard to be less nuts than the conquest of the entire western hemisphere) from a historical perspective.
Personally one of my first single player playthroughs will be a crazy world domination playthough as USA, to test AI and learn how all the bits of the game fit together. I forsee a lot of very interesting things happening in multiplayer games though...
To get back on the thread topic I think the biggest danger is Sweden running out of manpower. Or getting swarmed by the Soviets. Careful diplomacy is in order surely.