We “lost” the war in Vietnam due to several different reasons. There is never just one reason things don’t go the way we want them to. The United States lost the “war” by failing to declare an official war in Vietnam. It was declared to be a conflict but never a war.
Another reason is that the United States underestimated the enemy. They didn’t want us there, and we supported a South Vietnamese government which was very unpopular because of their repressive nature. We underestimated the determination of the North Vietnamese. They were determined to win.
We also lost because the United States leaders were not able to decide on the nature of the war, leading to confusion and strategies that proved to be ineffective. Our soldiers would attack and win land and then the politicians would pull them back and give back the advances we had just won.
The war was also lost politically because the American leaders were determined and committed to the struggle against communism and our pride prevented us from withdrawing our forces from Vietnam when we needed to withdraw. We also could never be sure of whom our enemies were. Both sides physically looked the same. It was difficult to look at a person and determine if they were North or South Vietnamese. Civil war is always difficult, and some still believe we had no business in the middle of that strife. Reminds many of what is happening in Iraq.
Yikes, I must comment because I was there and the word lost hits Viet Vets very hard. It's true that the politicians failed in Vietnam because like today they failed to understand the Asian mentality of war. The Vietnamese, once called the Tonkins, repelled Kubla Khan TWICE! They defeated the French and in WWII the Japanese decided that negotiation was the better option over fighting them for resources to run their war machine. With that said, the Viet Cong were farmers during the day but they also consisted of educated people, artists etc. who were "pressed" (conscripted) into service. They were tenacious fighters who had nothing but time. The NVA, Ho Chi Minh's National Liberation Front soldiers were uniformed troops who were definitely recognizable and formidable. The could travel for days with nothing more than a bowl of rice or hard Chinese biscuit. Some will disagree but for the youthful or naive among you please watch Forrest Gump, The Deer Hunter or my favorite: Apocalypse Now for the real truth about what we faced there. Jaymac, 3d Marine Division, 1966-1969.