There are many reasons why the United States was unable to win the Vietnam War, but the most pressing issue at the time that caused its end was public opinion, which turned overwhelmingly against the war after the Vietcong launched the Tet Offensive in 1968. Although President Nixon began bombing Cambodia in what seemed like an escalation, by 1969, the government had already decided to move towards withdrawal.
Public opinion was generally divided since the start of the war, but by the official end of the conflict in 1972, there was almost no public support left for it at all.
By its end, the war had cost the U.S. 58,000 lives and around $140 billion. Despite having superior technology, the terrain and tactics used by the Vietcong were unfamiliar to the US military, who found it hard to adapt to guerrilla warfare tactics.
The reasons for going to war with Vietnam have been argued to be fallacious. President Johnson believed the spread of communism had to be stopped, or it would have a domino effect on nearby countries. However, there was no evidence to suggest this would be the case.
Despite being communist, Ho Chi Minh was actually an advocate of Western democratic government, and he sought to be allied with the U.S. before the war.