The U.S. government viewed its involvement in the war as a way to prevent a communist takeover of South Vietnam. This was part of the domino theory of a wider containment policy, with the stated aim of stopping the spread of communism. The U.S. president at the time was Lydion Johnson.
The Chinese had the same idea. They also wanted to rule over Vietnam. They had a few meetings but couldn’t decide, and that had led to war. It was (a little) longer that everyone expected.
The Vietnam War (November 1, 1955 – April 30, 1975) was America's longest war and was fought in Southeast Asia. The twenty year conflict was fought between the Communist government of North Vietnam, who were supported by the Chinese, and the armies of South Vietnam, who were supported by the United States of America.
The United States, in line with their policy of Containment during the Cold War, were determined to prevent Communist aggression and provided the South Vietnam government with considerable aid and support. The US finally sent in troops to fight as well and the prolonged Vietnam War escalated in 1965.
The Vietnam War resulted in victory for the Communists when American troops were withdrawn in 1973 enabling South Vietnam to be completely taken over by communist forces in 1975.
The most immediate effect of the Vietnam War was the staggering death toll. The war killed an estimated 2 million people living in Vietnam, 1.1 million North Vietnamese troops, 200,000 South Vietnamese troops, and 58,000 U.S. troops.