Long before the Vietnam War broke out, the country had been in conflict and chronic political instability. Vietnam was divided in two between North Vietnam, a communist state backed mainly by the People's Republic of China, and South Vietnam, whose staunchly anti-communist regime was supported by the United States. After the French evacuated Vietnam, the country became a theater of conflict in the developing Cold War between East and West, with both sides offering support to their own Vietnamese proxies.
Over time, North Vietnamese communist guerrillas—the Viet Cong—became increasingly bold in their attacks upon the South. Under the leadership of Ho Chi Minh, they were able to carry out their operations using a 1,000 mile long trail—the Ho Chi Minh trail, as it became known—which stretched along the border with Laos and was protected by thick jungle cover. The Viet Cong's skill at jungle warfare would be a major feature of their war against the Americans.
In the United States, policy-makers were increasingly concerned with the in-roads the Communist insurgents were making into South Vietnam. It was widely believed that, if the whole of Vietnam fell to communism, then other countries in South-East Asia would quickly follow suit. This theory, the so-called "Domino Theory," led the Eisenhower Administration to step up support for the South Vietnamese dictator President Diem. To this end, several thousand military advisers were sent by the United States to train the South Vietnamese army to fight the Viet Cong.
Although the level of US support increased year by year, the Viet Cong continued to make substantial territorial gains at the expense of South Vietnam. It came as little surprise when South Vietnamese generals staged a coup against the corrupt, unpopular Diem and had him killed. Once more, opinion in Washington became concerned at the stealthy communist takeover of Vietnam. President Johnson ordered his generals to devise a contingency plan in the event of a full-scale civil war. He was prepared to send American troops to Vietnam, but only if it was seen that the United States was the victim, not the aggressor.
The Gulf of Tonkin incident provided Johnson with the opportunity to escalate American involvement in Vietnam. In August 1964, two American destroyers were attacked by North Vietnamese gunboats. In response, the US Senate passed the Gulf of Tonkin Resolution, which authorized the president to render armed assistance to any country requesting help in defense of its freedom. In March 1965, just seven months after the Gulf of Tonkin incident, the first American troops landed in Vietnam. By the end of the year, there would be 150,000 of them stationed in the country. The Vietnam War was now well and truly under way.