Canada

Currently we rank 28th of 145 in terms of military strength.

There's some reporting that may or may not be reliable (hence not linked,) saying that there's plans in place to fight against American invasion. Everything I've seen, and the little I've discussed with my friends in the Canadian forces, seems to suggest a quick response to buy time for the majority of our forces to arm and equip themselves, then to fight a guerrilla war. We're under staffed, underfunded (relatively speaking,) but highly trained, and extremely motivated. That's not taking into account the fact that there's a largeish portion of the population who are registered gun owners (roughly 25% national population,) the majority of those being long rifles. A history of an avid hunting culture across the whole country, means likely formation of militia groups doing ambush sniping, and lots of strikes on logistics and infrastructure.
We also have a lot of new citizens and PRs who were active in the Bosnian conflict, fighting for both sides. All this would make invading Canada initially easy, and then a long slow bleed.

But wtf do I know, I've never served, and I'm just a Stone Mason. Thanks for coming to my TedTalk.....