I've never heard it said better than this,
"Viet Nam was not the same thing as Iraq 1 which was not the same as Afghanistan which was not the same as Iraq 2.
Viet Nam was, at the end, a straight up infantry campaign in bad terrain, and in 1973 we demonstrated that we knew how to win that kind of war, and did. Decisively. Congress abandoned the military and the administration in 1975, or Saigon would be Saigon today. We at least learned not to use conscripts for a foreign war of attrition.
Perhaps we can learn from Viet Nam not to snatch defeat from the jaws of victory." - Jerry Pournelle 01/09/2006
Exactly what I think.