The United states thought that if South Vietnam fell to communism then other countries could fall too ,like falling dominoes. Their idea was called the domino theory. In August 1964 North Vietnamese torpedo boats had attacked an American ship. Then the Gulf of Tonkin was passed. This allowed the president to take any measures to get rid of any armed attack . As time passed the war in Vietnam escalated. More than 500,000 troops were sent to fight in Vietnam.