I think the issue can be explained without conspiracy theories. US military can't win against guerrilla, because US public cares about their image as the "good guys". And to be good guys you have to avoid any civilian casualties, which significantly raises the difficulty of all military operations.

The war against ISIS in Syria is a good example: Americans struggled for years, and then Russians came and pretty much ended the whole thing in 3 months. How? By bombarding civilians, including the hospitals, and (allegedly) using chemical weapons.

So, US have no problem winning the fight against guerillas, other than its own restraint.

Alternatively, some portion of the US public is against slaughtering innocent civilians, and it's not just about public image. I do agree with your general sentiment, though.

I still don't believe that brutality and disregard of civilians is the only way to efficiently win an armed conflict against insurgent groups, though.

This portion of the US public remain silent when millions of civilians were killed. So it's not a strong moral principle but just a preference.

— Hey, you! Don't kill civilians! — Okey.

— Hey, you! Don't kill civilians! — Идите на х! — Okey

US military can drop few nukes and end a war even faster. The weak joint is not a military. The weak point is cohort of bribed journalists, which write everything Russia want for small amount of money. When USA bomb civilians, bribed journalist will make woes. When Russia bomb civilians, bribed journalist will be silent or supportive. Why not to support state, which pays your bills?