You're thinking in terms of just US vs Canada. You're also forgetting that Canada is part of the Commonwealth. You can bet your bottom dollar that Commonwealth nations (at least a couple of whom are nuclear enabled, and some of whom have their own defence industries including ship and fighter jet production) would become involved. The US would find themselves on the short end of the NATO stick, including free trade agreements being jeopardised. Then there's the opportunism by Russia and China. Any distraction such as an invasion of Canada, and one that drags in allies, would provide an opportunity for them to apply global trade and banking restrictions, leverage geopolitical issues, disrupt supply chains, etc.
It would not be nearly as straight forward as "America military is big and grunts loud, Canada has beavers and mittens."
It would not be nearly as straight forward as "America military is big and grunts loud, Canada has beavers and mittens."