hope this is ok with the sub
I remember reading a few articles detailing how a military incursion into Canada would likely go.
tl;dr is that Canada's military would likely surrender immediately and then go underground to assist in resistance efforts. The Canada/US border is one of the longest unprotected land borders in the world and many cities along the border are right beside each other. The US would take immediate soft control but would be fighting fierce resistance coast-to-coast for decades that would frequently spill across the border and result in attacks on their own cities and facilities. Not to mention an assault on Canada is very unpopular with US citizens and troops and all of the states that border Canada will refuse to turn into battlegrounds. A lot of US citizens have family and friends in Canada and the militaries of the 2 countries have close bonds from working together for generations. If any countries came to Canada's aid with their own military force then it would get even uglier.
The immense economic strain of holding Canada against the resistance coupled with the internal unrest and inevitable worldwide sanctions placed on the US would cause complete economic collapse and eventual civil conflict. The USA, as the singular entity we know of today, would be destroyed.