From the WW II onwards
The Vietnam War was the longest war in United States history: promises to the people of South Vietnam to keep communist forces from overtaking them reached back into the Truman Administration. Eisenhower placed military advisers and CIA operatives in Vietnam, and John F. Kennedy sent American soldiers to Vietnam. Lyndon Johnson ordered the first real combat by American troops, and Richard Nixon concluded the war.
In 1941, a nationalist movement in Vietnam was formed and led by Ho Chi Minh. Ho was educated in the West, where he became a disciple of Marxist thought. Ho was resistant to colonial powers in Indochina (Vietnam, Laos, and Cambodia), he resented and resisted the French supremacy and had successfully won the hearts and minds of the majority of the Vietnamese people. The USA supported France against Ho Chi Minh from 1940s till its own military involment. The action of Americans was evoked mainly by the fear of domino theory, where after Vietnam communism could spread also in Laos, Cambodia, Thailand, Malaysia, Indonesia and Americans were not willing to allow so. In 1945-54 Vietnam was in war with France (supported by the USA) and after the French left, the country was divided by buffer zone at the 17th parallel into communist North (with Hanoi as the capital city and with new band of guerillas called Viet Cong created by Ho Chi Minh) and South (with the capital of Saigon) supported by Americans and led by unpopular Ngo Dinh Diem. 1964- 68 were the years of escalation of the conflict. North Vietnam boats fired on U.S. navy in Tonkin gulf. President Johnson got support in Congress to take all necessary measures. In the war there were only a few major ground battles, since mostly it was a guerilla war with the Viet Cong. Napalm was introduced to remove the jungle cover utilized by the Viet Cong and more bombs rained down on Vietnam than the Allies used on the Axis powers during the whole of World War II. Often unable to see the enemy through the dense growth of Vietnam's jungles, the U.S. military sprayed a chemical herbicide known as Agent Orange in an attempt to destroy the trees. Currently, debate rages on whether or not exposure to this compound is responsible for disease and disability in many Vietnam veterans. One of the most confounding problems faced by U.S. military personnel in Vietnam was identifying the enemy. The same Vietnamese peasant who waved hello in the daytime might be a Viet Cong guerrilla fighter by night.
One factor that influenced the failure of the United States in Vietnam was lack of public support. However, it lacked support only by the end of the war. Early initiatives made under Truman, Eisenhower, and Kennedy received broad support. Only two members of the United States Congress voted against granting Johnson broad authority to wage the war in Vietnam, and most Americans supported this measure as well. The antiwar movement in 1965 was small, and news of its activities was buried in the inner pages of newspapers, if there was any mention at all. However, in the late 1960s opposition against the war grew much stronger (average age of an American soldier dying in Vietnam was 19, compared to the age of 21 when young Americans where legally allowed to vote and drink alcohol; this influenced the law and the 26th Ammendment changed the suffrage to 18 years). Americans understood that immediate withdrawal would be a defeat, so in 1969 president Nixon introduced a plan later known as Vietnamization, which ment gradual withdrawal from Vietnam. In 1973 Paris Peace Treaty was signed among North and South Vietnam, the USA and Viet Cong and in 1976 Vietnam united as Vietnam Socialist Republic. Neighboring Cambodia and Laos also became communist dictatorships.