Except both those examples are of guerrilla warfare in unfamiliar locations where its extremely time and resource intensive to get supplies.
None of those restrictions would apply if the US military had to engage on its own soil where they A) Have perfect information and B) Are arms reach away from all their ordinance and resources. So while they look similar on the surface theyre actually nothing alike, at all.
I think it has more to do with the fact that from start of the 20th century a stand up army has slowly lost its effectiveness in conquering a country. Which is why if you wanted to reap the benefits of taking over a place you'd do something like, buy up real estate in that country, maybe rig their elections, create a dependency on you for aid.
And one of the main reasons for that is because a gun is a equalizer. An individual with virtually no weapons training can murder dozens, potentially even hundreds, just by having a functioning trigger finger. And a terrorist group with access to the internet can learn to make bombs in their basement.
In response to your rationalization for why Vietnam and Afghanistan were different. Germany invaded France during WWII, a country geographically not that different, a country they're familiar with from previous wars (To the point that many serving in high ranking positions in the military had first hand experience fighting there), and a country close enough to their main resources and command. But, the French Resistance formed, made up of armed and educated members of the public, and I'm willing to bet that if the war had proceeded the resistance would have lasted decades.
I want to make it clear that I'm in no way of the belief that a well armed militia is necessary to rise against the tyrannical government. I think it's an irrational fear.
And, I hope I'm not coming off as combatant, this is all in the spirit of engaging conversation.