Canada

At best, Canada would be able to do guerrilla type cell warfare. But Canada itself would be fucked.

Think about the recent announcement to do with building a "Sovereign Canadian Internet". That move highlights, quite explicitly, how we effectively don't have data cables running east west across Canada, nor do we have significant Canadian data pipelines to other nations -- we use the American backbone. So even our basic communications east / west across the country, for coordinating actions, would be obliterated in like the first hour of a serious conflict. Land lines are practically archaic at this stage, and every Canadian government institution is reliant on Microsoft, and US services/backbones. In the first hour, that's all toast. Almost all of our banks and financial institutions, and their regulators, are reliant on US service providers to function -- our ATMs run on fucking windows -- so all that goes poof.

Yes, a small cell with a modest drone setup and a chicken coup in the woods could develop bio weapons and spray it all over American cities, or do something like cow-ebola and spray their mega farms to fuckup their beef supply. We could inflict pain, yes, but we'd lose the overall conflict very quickly in terms of our institutions and country being reduced to shit.

Even more, Id also argue that Canadians wouldn't generally fight back. It's not like regular Canadians own a shitload of guns, like in the states. Even more, a huge number of Canadians identify more with their country of origin, than with Canada -- Trudeau even bragged about how Canada was the first "post national" country with no fuckin identity/cohesion, as though it was a good thing. Our population has been dependent on immigration for growth since like 2000, with a huge percentage of our younger people not really giving a fuck about "Canada" as a country. How much resistance do you think something like the millions of newly immigrated Indians between the ages of 18-30 are going to step up and fight America? Hell, many of them wanted to use Canada as a stepping stone to get American visas/citizenship anyhow, so an invasion speeds up their plans / helps them. There's no reason to think that people without a history in Canada would fight to protect it en masse.

Bridge outside their town?

If US invades us I'm bringing the fight a lot fucking closer to home than a bridge.

Dream on.

Americans would react similarly to how the general citizen of Russia has acted during the Ukraine conflict. So long as their regular lives aren't disrupted by the conflict, the vast majority would shrug and not give a shit. Or, they'd do exactly as you just noted, and say "This is a political problem that can't be solved with violence (without creating much much more violence and harm in the process)".