Fighting in someone else's neighborhood (Iraq and Afghanistan), regular U.S. Army and Marine units, not Spec Ops, have been kicking the living "sierra" outta some very determined fighters for a couple of years now. Set aside any rhetoric about whether we're really changing anything there, I'm just talking about the overwhelming U.S. military success.
Now, move 'em back onto their own homeland with far easier resupply capacity, friendly locals who would be glad to have the U.S. military fighting a foreign invader, and toss in Spec Ops waging a fight against the invaders.
China and Russia are about the only ones with the capacity to invade us, and it would so tax their supply lines I don't think they could pull it off.
One more likely scenario would be for either China or Russia to invade Canada, claim land there, and hold it. I think they could do it with ease, militarily. Politically, who knows. The Old Bear has shown signs in recent years it's regrowing it's once clipped claws from the days of the Soviet Union.
Once they had a toehold in Canada, then they could try and start amassing tanks, soldiers and weapons, but we'd see it coming a thousand miles away. We saw it in Cuba, and Kennedy faced down Khrushchev over it.
Another scenario would be a coalition of Russia and China invading. Again Canada would fall easily, but then I think Canada could be taken with a few hundred THR members. Russia and China invading the U.S. directly? Hmmmmm, maybe.
A third scenario, and one which I think has the greatest chance of success, would be an invasion of the U.S. after the current 50/50 nation splits into two or more Americas in a bloodless secession of states. The weaker, or weakest, of the Americas would be the easy target, and even easier for both Russia and China.