Canada

Canada is five large cities, which are easy to capture, and an entire USSR-size back-country of rough terrain, which would be nearly impossible to patrol and hold in the face of even a mildly resentful populace. You don't have to have the entire population actively fighting - passive resistance would make it difficult to do anything in the face of shrugs and "I dunno"s.

Resistance fighting can be picked up pretty fast, and in the face of brutal American reprisals and repression, I think it would be. Their history in Afghanistan and Vietnam shows that they'd create an entire generation of people out for revenge VERY quickly.

We would hide our insurgents, and they would fade into the mountains and forests. You don't need a lot of active insurgents to cause a huge problem.

We have cheap drones and abundant weapons. We're one of the more gun-infested countries per capita, and have a porous border with the USA.

It'd be a hell on Earth. They'd never hold the Rockies.

Resource extraction operations. Forestry and mining are easy to disrupt, especially if the local labour doesn't like the occupiers. There are a thousand ways to gum up the works, and a thousand more to attack them and fade away into the surrounding forest.

Yeah, you'd think it's easy for the US.

Just as people thought Iraq would be easy

And Afghanistan

And Vietnam

And yeah, the initial invasion is easy, it waltzes over anything as a hot knife slices through butter.

Then, however,.comes occupation. What is it worth invading and blockading if you can't own it? And this is where the US always has failed and will continue to fail in the future too. You can bet your furry Canadian butt that Canadians will resist and hard.

Same goes for Mexico btw. If the US decides to invade there as hinted as multiple time, expect US soldiers to be hanging from bridges with their private parts stuffed down their throats, they have quite a bit of experience with how to handle that.