Many people wonder why the United States fought in the Vietnam War. The reason why we fought in Vietnam was mainly because of Communism. President Eisenhower had come up with a theory about Communism called the Domino Theory. The Domino Theory stated that if a country fell to Communism than the surrounding countries would also fall to Communism. The United States was afraid that if Vietnam had fallen to Communism that they too would also fall to Communism. Communism terrified the people in the United States. During Eisenhower's presidency, which was after World War II the French, had controlled Vietnam. In 1954 the former French colony of Vietnam was divided at the 17th parallel into two parts. North Vietnam, the Communist country, and South Vietnam, the anti Communist country. The Vietnam Communist Party controlled North Vietnam and Ngo Dinh Diem controlled South Vietnam. The United States supported South Vietnam. The Viet Cong (Vietnam Communist) tried to over throw the Diem government in order to gain control of S. Vietnam and turn it into a Communist country also. In 1963 a group of Deim's own soldiers from the South Vietnamese army killed Diem and overthrew his government. This event led to the Vietnam War.The U.S. knew that Diem's own people were going to over throw him. President Johnson had to send troops to Vietnam or the people would think that he was soft on communism. Also Johnson wanted his Great Society passed by Congress he had to send troops to Vietnam. Johnson had known that the United States could never win a war in Asia even before he sent troops to fight. In 1968 more than a half a million American soldiers were sent to Vietnam yet Congress had not declared war on North Vietnam. President Johnson's plan to esculate the war over three years had worked.