Answer:
As others have stated, the US didn't lose the Vietnam War in the traditional sense. A peace was negotiated by Kissinger and the N.Vietnamese representatives in Geneva, after neither side was seeing progress. The US then pulled itself fully out of S.Vietnam after the Fall of Saigon: the embassy was taken, and many, many diplomats and refugees were airlifted to the USS Midway.
The reasons for the failure of the US in the war are numerous, however.
Firstly, let us look at US military itself. Having not learnt from the French defeat at Diem Bien Phu in 1954, US troops were poorly trained for guerilla warfare; most training happened over a few weeks in Texas or Alabama, hardly representative of the Vietnamese jungle. Of the 2.8 million troops who served, 2 million were conscripted; in the end over 500,000 soldiers deserted, and most were unwilling to fight-only 10% ever saw combat. Due to the lack of career soldiers and the yearly 'tour of duty' only 6% of deaths happened in the final three months, as troops tried to stay clear of danger. By comparison, due to their inexperience, 43% of deaths occurred in the first three months of duty, which was not helped by the reception the 'cherries' received from their fellow men. If they survived, drug abuse was rife: in 1971 5000 men were admitted to hospital for combat wounds, but 20,000 for drug abuse. Officers were not liked either: 3% were killed by their own troops.
Explanation:
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