AIUI the US typically doesn't declare war any more. Korea, Vietnam, Grenada, Kuwait, Afghanistan, Iraq, Libya and so on were police actions or something similar.Only Congress can declare war, not the president. If Trump ordered troops to invade a foreign country, that would be an illegal act of war. Subsequent approval after the fact would be seen as condoning natiowide illegal activity. It would be the equivalent of Russia's Duma condoning Putin's ventures into Ukraine.
But yeah, this GOP would do it.
Also, the Canadians are perfectly capable of striking back...hard. Like Ukraine, they have a fair contingent of modern US made weapons, including aircraft.
For Canada Trump could declare a Fentanyl emergency, use the military to attack some "labs" in Canada and then go all in to "protect" the military once Canada defends herself.
Look at Hitler's approach to taking Czechoslovakia.