Well in the case of the places Trump wants to invade Canada and Greenland a war would very likely be a short special military operation. The US can easily blockade both land, sea, and air, so they don’t get any external supplies. After a short air campaign and ground invasion, the Canadian armed forces would have to retreat north outside the main population zones. They would run out of supplies pretty quickly. An ensuing guerilla war is possible, but unlikely to repel American forces any time soon.

The diplomatic fallout would be pretty bad though.