Democratic Party partisans like to remember Vietnam as Richard Nixon's war. Clearly that view is merely propaganda. Richard Nixon ended American involvement in the war, accepting the political consequence of a defeat of the United States and its South Vietnamese allies by the communist party of Vietnam. It is true that the peace plan Nixon had when elected in 1968 did not work out (basically, peace through victory), and a lot of people died needlessly during further years of bloody fighting, with U.S. troops involved until 1973. But it was Richard Nixon's war by inheritance only.
Former president Lyndon Johnson gets most of the blame for the Vietnam War. He deserves it. He was, of course, the leader of the Democratic Party, so its partisans like to try to pretend he did not exist, except when you start talking about Civil Rights. Lyndon Johnson was the first Democratic Party President of the United States who really fought for civil rights [possible exception: Harry Truman].
Lyndon Johnson became President when John F. Kennedy was assassinated on November 22, 1963. At that time the United States had a major presence in South Vietnam, mainly providing economic and military aid to the puppet government of Ngo Dinh Diem. But the U.S. already had 16,000 military personnel stationed in South Vietnam. We had a naval fleet off the coast of Vietnam (including off North Vietnam) and a large military presence in our nearby former colony, the Philippines. While Diem was a Vietnamese nationalist who did not like taking orders from the U.S., his ability to fight to retain his hold on South Vietnam was nearly totally dependent on U.S. aid. So in effect South Vietnam had become a U.S. colonial possession with a government that was semi-autonomous. Just like under the old French colonial system. Lyndon Johnson and the U.S. Navy faked the Gulf of Tonkin incident in order to justify even greater U.S. military intervention. When Johnson left office in early 1969 the U.S. had 550,000 troops on the ground in South Vietnam and was bombing North Vietnam on a daily basis.
Democratic Party partisans hate it when you blame the war on John F. Kennedy. They like to say that if Kennedy had lived he would have with withdrawn U.S. troops from Vietnam. Some conspiracy theorists believe that Kennedy was getting reading to pull the plug; that was why he was assassinated. It is not that there is absolutely nothing to support this view, but the support is thin. The basic facts speak for themselves: when Kennedy entered office the U.S. had 500 military advisors in South Vietnam. When he died he left 16,000 U.S. soldiers there. That sounds like a solid escalation of the conflict to me.
But hey, most sources put the number of U.S. military men in South Vietnam when Ike left office in 1961 at 500. I heard a Democratic Party hack say the Vietnam War was a Republican Party war because Dwight Eisenhower was the first to send military advisors to Vietnam. This takes us back to 1953, when a variety of Vietnamese nationalist groups were trying to oust the French colonial rulers. The communists, or Vietminh, were the largest group and were the only serious military threat to the French. The U.S. was providing most of the money and military material that enabled the French army to fight. Then in 1954 the Geneva Conference was held. The French agreed to withdraw from Vietnam. A non-communist but nationalist regime would take temporary control of the South. The Vietminh took over in the North. A nationwide election was to take place within 2 years. The Vietminh would have easily won a nationwide election, but might have allowed non-communist parties to be part of the government.
The U.S. and France, however, were determined to stop communism from spreading. The gave the governance of South Vietnam to Ngo Dinh Diem and his crew. No national elections were held. The communists were not particularly strong, militarily, in the south. U.S. aid and a handful of advisors, along with continued French ownership of important businesses, were all that were needed during the Eisenhower years. Of course Richard Nixon was Eisenhower's Vice-President, and was as anti-communist as the Kennedy brothers were at that time. It should be noted that one of Diem's big supporters in the U.S. in the 1950s was Senator John F. Kennedy.
But the whole idea of aiding the French to defeat the Vietminh goes back to Harry Truman. Both parties and the media blamed him for losing China to the communists. He (and most high-ranking Americans) saw communism as monolithic. He did not want to involve U.S. troops. So he paid the French to fight the Vietminh, which they were happy to do, given the economic benefits to France of retaining their colony.
To add some irony to the situation, Vietnam had been one of the sore points that had led to the conflict between the U.S. and Japan in World War II. In Vietnam during World War II the French were happy to follow the orders of Hitler's puppet Vichy regime in France, and Hitler's orders were to do what the Japanese occupiers said. So when the U.S. was trying to get guerillas to fight Japan, which in Indochina meant fighting the French army, it trained and armed -- the Vietminh! Then, instead of punishing the French in Indochina for siding with the Nazis, the U.S. backed their continued control of the colony!
So which president is to blame for the Vietnam War? Take your pick. Or think of it this way: the U.S. has a ruling elite. The elite rules regardless of which party is in power. And a policy, once set, can go on for decades no matter how stupid or inhumane it is.
recommended reading: Stanley Karnow's Vietnam, A History