There’s a huge difference between a country waging a foreign war and a country waging a war against its own cities and citizens.

Indeed, in that the country isn't generally going to have the will to massacre its own citizens. So it's not that the citizens have a gun that is staying its hand, it's the lack of desire to just go ahead and slaughter them. Part of that is going to come from the fact that the country's military is made up of people who have these citizens as their friends, neighbors, etc.

So having a gun isn't helping you at all in that situation.

Insurgencies work and has been proven time and time again. See: Iraq

So these insurgents control Iraq now? They "won"?

Also, it's not really a comparable situation: dealing with insurgents halfway across the world is harder than dealing with them in your backyard. The US was also not really trying to take over and just rule the country, they were at least half-heartedly trying to set up a government that could manage stuff. In addition, the middle east and oil is strategically important to the US but insurgents in Iraq are obviously much less of a pressing concern than insurgents in the US. Naturally the US isn't going to invest the same level of resources and go to the same lengths to deal with those problems in Iraq compared to what it would do if that was happening in its own backyard. Finally, a lot of the insurgents were religious extremists. Even if every liberal in the US was armed, do you think we/they would really go to those lengths? I think we're just too used to living comfortably and don't really believe anything with that level of fanaticism to be running around with an AK waging a guerrilla war even if we had the AK.