The reputational damage would be hard to understate. I think anyone with a bone to pick with the States would happily and repeatedly point to us Canadians as an example of what happens when you trust Americans. And that would be enough to achieve a lot of their regional policy goals, if not much more. Mexico would probably seek other political partners to make sure they're not next.

Yes, an American invasion of Canada would immeidately set the stage for a worldwide rationale for opposing US interests.

Annexation of Canada would mean the end of the soft power for the Americans. The only way anyone would let them in would be through hard wars.

At least China and Russia only forcibly attacked independent countries that were once united with them.