For both the US and the country it invaded, the differences between the Vietnam and Afghanistan wars are minimal. From Patrick Armstrong at strategic-culture.org:
The difference between the U.S. performances in Vietnam and Afghanistan is that in the first, the vehicles were painted green and in the second, sand.
The lesson of Afghanistan is not that the US is washed up as a great power. The lesson is that the US is such a great power, militarily and economically, that it is continually tempted to try hopeless things that nobody else on earth – including China – would ever attempt.
Don’t believe what you’re told by the generals, or the ambassadors, or people in the administration saying we’re never going to do this again. That’s exactly what we said after Vietnam. We’re never going to do this again. Lo and behold we did Iraq. And we did Afghanistan. We will do this again.
Bill Ehrhart arrived in Vietnam in 1967 believing everything. His first indication that all was not as expected came when he wasn’t welcomed the way Allied soldiers had been in 1944. A couple of days later he was shocked to see “detainees”, bound hand and foot, casually tossed off a high vehicle by other Marines. This didn’t seem to be the way to treat people the Americans were there to help said he to his companion who told him to keep his mouth shut “until you know what’s going on around here”. And, he continues in this video, “it went downhill from there”. Every day patrols met “snipers and mines” but he saw hardly any enemy soldiers. He came to realise that the Viet Cong didn’t have to force people to fight the Americans; once a Marine patrol had destroyed its way through a village, they’d have all the recruits they needed:
The war he saw bore no resemblance to the optimistic stuff he read in Time Magazine and other mass media. So he hunkered down, stopped asking the questions of what and why – “the questions themselves were too ugly even to ask” – did what he did and waited for the date when he would go home.