He's so far outside the box predictions don't work anymore. But food for thought; Americans associate drugs with Mexico far more than any other country. So this would be one hell of a head whip for the general public. Americans also still consider Canada one of our closest friends on the global stage. Many families in the North span the border. For that fact alone there's a very good chance that if he attacks Canada we'd shut this country down with protests and strikes.
On a rational predictable level he's already declared cartels to be terrorists and Venezuela to be a terrorist state. This is very likely another tool to pressure the Mexican government.
That said, I wouldn't trust us right now. We're like that friend who took too much cocaine and is hiding in the closet with a gun. Unpredictable and dangerous.