It would be a shit storm of unimaginable proportions. The US would be attacking another NATO nation and Canada would almost certainly invoke Article 5. Those not assisting militarily would probably sanction the shit out of the US and many of their previous treaties, pacts, and agreements would be withdrawn.

Recent polling in the US has shown that US citizens are greatly opposed to the ongoing trade war with Canada and would be even more opposed to further conflict. The executive branch ordering an attack on a historic ally and neighbour would cause catastrophic rift in the US military chain of command and I’d imagine many would disobey orders. States along the Canada/US border would likely be fiercely opposed to having their states turned into battlegrounds and would resist legally in whatever way they could.

Canada does not have the military for a large prolonged conflict, especially from our closest ally. An actual war would probably be quite shortlived, but it would also probably be the last thing the Trump administration ever does. The economic and civil-unrest blowback would rip apart his administration. Ordering your citizens to kill their neighbours, friends, and even family is not something you do without major consequences.