How precisely Donald Trump could make good on his threat to annex Canada can be found in the U.S. Constitution. There is both potential and precedent in American history.
theconversation.com
Not impossible, not unthinkable
Every Canadian needs to pay attention to this bit of American history. In one treaty, the U.S. annexed the present-day states of California, Nevada, Utah, New Mexico, Arizona, Colorado, Oklahoma, Kansas and Wyoming. It subsequently illegally invaded Indigenous territory in the west.
Canada could be next — perhaps not immediately as the 51st state, but quite possibly as a U.S. territory that would deny Canadians any voting rights for Congress or the presidency, allow only some autonomy and make questions of citizenship ambiguous. The constitutional architecture exists in the U.S. to make it happen.