President Eisenhower believed in what's known as The Domino Theory - the belief that if one country in Southeast Asia gave into communism more countries would start to follow suit, much like falling dominoes. Eisenhower said in his speech "This would lead to disintegration in Southeast Asia, with the "loss of Indochina, of Burma, of Thailand, of the Peninsula, and Indonesia following." " (http://www.history.com/this-day-in-history/eisenhower-gives-famous-domino-theory-speech). This was Eisenhower's reason for sending soldiers and aid to South Vietnam. .
Later, during Nixon's administration, the North Vietnamese began bombarding the south with a series of surprise attacks in what's known as the Tet Offensive. "It was considered to be a turning point in the Vietnam War (http://www.u-s-history.com/pages/h1862.html). In reaction to this Nixon authorized "vietnamization, which was a plan to help build up the South Vietnamese army, and prepare them for taking on the North "in order to facilitate a gradual withdrawal of U.S. troops " (http://www.history.com/topics/vietnamization). And finally, during Lyndon B. Johnson's presidency, the U.S. began to fight in the war because it was believed that the Vietnamese attacked one of our ships while we were patrolling the Tonkin Gulf. This set forth the Tonkin Gulf Resolution, passed by Congress and giving Johnson the authority to do whatever he thought necessary to stop the Vietnamese. .
Most people know the Vietnam war for the protests and chaos in America during the war, but what many don't know is the "protests did not start when America declared her open involvement in the war in 1964 the first protests came in October, 1965 when the draft was increased; " and after that there was protesting everywhere in the U.S." (http://www.historylearningsite.