He explicitly said he would use "economic force" to conquer Canada, meaning he would break our economy and drive Canadians into poverty in order to force us to join the US. The Idiot Senator from Maine has stated how that would go - no shadows of Canada would be allowed to remain. The violence would implicitly come afterward, because the vast, vast majority of Canadians would not be okay with this - especially since we would not be allowed to vote as states, since Canada is by nature a "blue state" and Republicans would not allow more blue votes. There would be protests, perhaps even Troubles-style terrorism, and Trump would send in the military just like he is currently doing for other uppity "blue" areas like DC and Chicago.
He wouldn't call it "military". He'd call it cracking down on crime, just like he is now. But people would die... probably a lot of people.