but you'd think Americans would've got that point by now after losing the Vietnam War and pulling out of Iraq and Afghanistan.
Vietnam - At the time it was two nations, both on the far side of the world's largest ocean, one of which (the northern one) the US decided not to engage in ground operations in, and all whilst said nation was supplied by not one, but two major powers (one of whom was the only other superpower); and despite these limitations, South Vietnam (the nation the US supported) weathered the Viet Cong, and NVA till US aid was cut in 74 even after the last US ground troops left.
Iraq - Not sure what your point is here, US still maintains bases in Iraq, and the current government is allied with the US.
Afghanistan - Yet another nation on the far side of the planet from the US proper, oh and its landlocked, is either desert or very mountainous, and possessed of a highly militant population that up till that point had been engaged in armed conflict for 21 years (1979-2001). Even than the US owned nearly every inch of it for twenty years, and we only left because we got tired of trying to turn tribal savages into first world citizens.
None of these wars paints a picture you think it does. All represent military actions of the US on the far side of the planet, not ones in a nation that shares a land border with the US. One war involved billions in military aid from the only other superpower at the time (USSR), and the others occurred against nations/peoples who are/were a thousand times better equipped and more war like then modern Canada. All involved large loses for Americans enemies for the amount of Americans they killed. All lasted for years, none involved a total war policy on the part of the US like during WW2.
America isn't going to annex Canada, but not because we can't do it. And the worst part is annexation is in y'alls best interest more than it is ours.