The guerrillas fighting the German occupation of Europe are not an example of failed guerrillas or an example of guerrillas who had no state backing, so I don’t see how they support the point you are making. They, if anything, received the greatest of all state backing by the Allies and were capable of assassinating key figures in the Third Reich such as Reinhard Heydrich. That was not a civil war, but resistance to a foreign occupation with the ultimate aim of aiding the Allied war effort, so it is not what I was talking about anyway, and if anything it succeeded in its aims. Resistance to foreign occupation can compel occupiers to leave a lot sooner — the Islamic Jihad group killed over 200 US soldiers with simple bomb trucks and this helped force the US forces to leave Lebanon. But that is not what I was talking about here. In civil wars, foreign states (and foreign non-states) have always gotten involved — as France did in the US War of Independence. Attempted foreign involvement occurs in every civil war, it is only a matter of degree. I never said insurgents win in a “closed system”. Civil wars are international by definition, because the world is a single social system. In fact, I acknowledge this in the article itself:

Add the internal economic devastation caused by citizens committing acts of sabotage and civil disobedience, foreign sanctions by other states, and even international aid to the insurgents by external actors, and the tyrants could be ousted even by the most lightly armed militia units.

Expecting hypothetical anti-US guerrillas not to get backing from a foreign state is like expecting no-one to bid on the single most valuable are rare product on eBay. Why would foreign states miss the opportunity to remove what most people see as the most dangerous and mighty regime in the world? This doesn’t mean we should portray civil wars as somehow always being started and coordinated by a foreign state/entity, although clearly that can happen. Foreign states are opportunistic — they bid on existing foreign guerrillas, and that massively increases the likelihood that the guerrillas will win. That is the nature of politics.

I believe it’s not naive to say the US would have even more weaknesses than most states if it faced a civil war evolving from broad civil unrest. The trend of “insider threats” like Edward Snowden in the US is deeply feared by the US government itself. This opens up the possibility that “insiders” would give so much assistance to the insurgents anyway, it could theoretically remove the need for foreign backing at all. Add in the number of US civilians who actually have great military experience, operate private security firms and have a lot of technology at their disposal, the abundance of oligarchs and millionaires, and the possibility that a fair number of them would be on the sides of the insurgents too even if most state elements stayed loyal to the state.

Also, you avoid the part where I stated that the US government likes to bomb foreign infrastructure but would not want to obliterate its own infrastructure just to fend off guerrillas. If militants were firing mortars at the US Capitol, the US would not send F-16s to bomb entire towns or unleash white phosphorous on its citizens. It would send SEALs. So then the fight becomes purely a question of whether the SEALs are better killers than potential ex-SEALs who could be the ones with the mortars. If the US did use the F-16s, it would only cause more US people to join the insurgency and foreign states to take a keener interest in events as an international scandal grows.