Canada

There would be a problem of "you go first…"

If the US became Imperialistic, and I mean old-school Imperial, non of this new age cultural or economic imperialism, then they could probably "take" their first conquest or two without much opposition. In this case Canada and Greenland would probably fall to US occupation.

It's what would happen next that would destroy the American Empire. Everything you said is true, and every one of the US's allies would wonder if they are next. If Canada, the US's closest ally for generations cannot trust the US who can? The clear answer is no one. I would expect a swift unravelling of the global order. The EU would be an interesting case. With a belligerent (but failing) Russia to the East, and their long standing guarantor of peace suddenly no better, would it crack under the stress or would something more capable emerge?

It's a horrific dystopian future we face, the entire globe would be plunged into an era of increased tension and military spending in the best case.

The current US "empire" has shown that buying something is cheaper and more beneficial than conquering something. In a few short years Trump might effectively vastly increase the cost of power from Canadian hydro facilities and at the same time scare away any others that would do business with them.