Is there any historical, legal, or strategic precedent that would make this even remotely feasible? And given the likely short political shelf life of the current U.S. administration, would any of this outlast the next four years anyway?
Then again, certain governments continue to act like we were still in the XIX century so "might makes right" (Russia, Israel, China, Morocco, Turkey...). If one is not ashamed to be in such an esteemed company, everything is possible.
They've tried twice already, the second time the Canadians (/British at the time) burned down the White House: https://en.wikipedia.org/wiki/Burning_of_Washington
And Bush still destroyed it with ease. The idea that the USA couldn't annex Canada by force is just silly.
We'd fight, and slit your throats in the dark. But there are some of us who'd just roll over, or welcome you with open arms. Just look at the morons in Alberta talking USA annexation -- they're probably 10% of the population. That's enough for a Quisling regime.