U.S. invasion of Canada
Due to the balance of power effects internally to the US, and the propaganda effects of invading such a peaceful neighbour, the idea of U.S. invasion of Canada is impractical and ridiculous. Despite this various commentators in American mass media make references to it as it if were actually possible, e.g. Tucker Carlson.
In fact, the idea is so ridiculous it is less likely than a Canadian invasion of the U.S.:
First, Canadians are so similar to Americans and have so many family ties across the border that it would be absolutely impossible not to let Canadians vote or not to recognize Canadian provinces (at least the big ones) as full status states. An Israeli-style occupation without votes would not work. Accordingly:
Republicans would not invade and give the House, Senate, Presidency and Supreme Court permanently to Democrats by giving Canadians (very progressive) votes, and creating new states that would certainly be overwhelmingly Democratic. Nor would Democrats invade the only country they can point to for good examples. Neither party would actually want to try to explain trying this, to the world, either. Mexicans in the Southwest might seize power in some places, fearing an attack on themselves. And, Governors jealous of the powers Canadian provincial Premiers now have, and would be likely to retain in any peace deal, would want to become Provinces, rather than having the Canadian provinces become states.
Perhaps the most important reasons the US cannot invade Canada, however, is that the Bush administration cannot find it on a map, and cannot find Republicans near Canada to ask directions from.