My point is that American civilians, have military style weapons. LOTS of them. A 2.23cal/5.56mm round, is a NATO round.
With all those weapons floating around, there could be no land invasion. @seawulf575 has a point. We could be nuked, or something. But , NO country, could go door to door. They’d suffer a crazy amount of casualties. I could probably kill/injure, at least 3–5 enemy occupants. That’s just one guy, in one house. If you look at the numbers, that starts adding up fast. If I saw it coming, I’d take my weapons, and such, into the swamp. I’d be far more effective there. I might could take out way more. Remember that the British tried to find the Swamp Fox, in Hell Hole Swamp? That’s where I fish… That’s my back yard son. Alligator infested, and tricky terrain. That’s where I would go.
An invading enemy, doesn’t ususually want to destroy where they’re invading. Their best bet, would be chemical /biological weapons. I bet you I could hunker down, in the swamp, with a gas mask (yep, I have one,) and I’d probably die, eventually, but not after taking out a bunch of them. That’s just me. Losing 20 soldiers per each southerners, would add up.
And they’d have to go through the Pacific Ocean, Arctic Circle, or otherwise, just to get here… Canada, probably couldn’t fend them off. But , they would put up a fight.
I just don’t see how a foreign invasion, could work.