The United States entered the Vietnam War to prevent the spread of communism, but foreign policy, economic interests, national anxieties, and geopolitical plans also played important roles. The United States was the only country that fought on both sides of the conflict, so its involvement can't be explained by any single factor.
The French began the war by bombing Vietnamese communist targets in December 1955, but they were soon forced out by the North Vietnamese. American officials became concerned that if Vietnam fell to communism, then China might eventually take control of the region. The United States therefore had an interest in preventing a Communist takeover of Vietnam; this reason is called the "domino theory".
The Soviet Union supported North Vietnam, helping it gain military resources and equipment. This support included providing weapons and training for thousands of Vietnamese soldiers. In return, North Vietnam provided material assistance to the Soviet Union's own struggle against South Vietnam and America. This reason is called the "trading partner" theory.
The United States pulled out of the Vietnam War in 1973 after the fall of Saigon. However, it didn't stop fighting - it continued to work with other countries to reduce violence between Vietnameses and non-Vietnamese fighters.
Many reasons influenced the United States' engagement in Vietnam, including ideology, Cold War strategy, and inheriting a colonial heritage from the 4th Republic of France, one of its primary allies. There were two key motivators: anticommunist and anticolonialist sentiments. The United States was opposed to communism's rise as a global phenomenon and saw Vietnam as an important ally in promoting freedom around the world.
As early as 1785, the first American ambassador was appointed to Vietnam. In 1945, after World War II, the United States played a major role in helping France restore its sovereignty over its former colony. Under the terms of the peace agreement, Vietnam was supposed to be a constitutional monarchy but instead it became a communist state in 1946. The United States maintained diplomatic relations with Vietnam until 1955 when the government in Hanoi refused to renew our license for an embassy there. Since then, we have had no formal presence in Vietnam except for a few chargés d'affaires and military observers stationed in Saigon.
In 2001, President George W. Bush signed a decree authorizing the establishment of full diplomatic relations with Vietnam. This means that the United States will now have a mission in Hanoi and if requested, a visa section at the American Embassy in Bangkok.
These are the only two countries in Asia that do not have official diplomatic ties with the United States.
The US entered Vietnam with the primary goal of preventing a communist control of the region. In that regard, it failed: in July 1976, the two Vietnams were unified under a communist flag. The communists also took over neighboring Laos and Cambodia. However, the US continued to fight in Vietnam until 1973, when it was defeated by the Vietnamese army. This final withdrawal came after a series of secret negotiations between the US and North Vietnam led by Henry Kissinger. The agreement called for an end to US involvement in the conflict and the withdrawal of all American troops with time for them to be replaced by South Vietnamese allies.
At first, the US had no intention of leaving Vietnam alone. But then Nixon declared that America would not be "the first nation to run away from war", and so he ordered the departure of all US forces by year's end.
After many years of fighting, this short-lived peace agreement proved to be just that - temporary. Communist leader Ho Chi Minh refused to honor his side of the deal, and so the war continued. By the time Carter became president in 1977, there were already nearly 20,000 US casualties and billions of dollars spent on the war effort.
It is estimated that between 464,000 and 794,000 people died as a result of the war. Many more were injured or made homeless by the fighting.