What started the Vietnam war?

Vietnam was a problem the United States inherited after the French were defeated ending 150 years of colonial rule and the United States at the height of its post war power had to mediate a solution. The Geneva Accord of 1954 partitioned the country into North and South (a two year interim) until elections could be held to determine a leader. It clearly stated that the line was “provisional” and not to be interpreted “as constituting a political or territorial boundary.”
When it became clear that the popular leader, Ho Chi Minh would be overwhelmingly elected, the U.S. never allowed the election to occur. He was a nationalist and while studying in Paris became influenced by Marxist teachings, although he asked many times for American help in setting up a government modeled on the American type.
Meanwhile the American backed Diem regime in the south became unpopular and ineffective in dealing with the political and economic realities of governing. Buddhist monks starting themselves on fire in public and guerilla incursions from the north demanded some type of action. The problem was framed not as a national liberation event, but as another cold war, stop the communist advance problem.
Kennedy’s instinct was caution and was wary of the military option, however, he sent advisors to try and stem the problem. It soon became clear through the New York Times writers David Halbestam and Neil Shehan that we were being lied to when an aircraft carrier was spotted at the mouth of the Mekong Delta and advisors were being killed in combat action. To escalate or withdraw was the Kennedy conundrum.
A few generals had warned not to get involved in a land war in Asia, including Gen. MacArthur who fought two of them. Also we were inexperienced in the guerilla warfare arena especially since we were the foreign army and lacked the intensity of an army defining its homeland. Another obstacle, the South Viet Nam army was lukewarm in defending a government not to their liking. It appears in speeches that Kennedy wanted to withdraw and was postponing the decision until after the election so as not to appear as soft on communism.
Our next president, Lyndon Johnson, was less temperate. The Gulf of Tonkin incident – an American ship being fired upon - was the trigger point for a congressional vote of 98 to 2 to go to war. The event has since been agreed to as being fabricated.

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.