The United States became involved with the Vietnam War to directly combat the rise of Communism, which had become the de facto governmental structure during turbulent times of internal war. As South Vietnam tried and failed, several times, to institute a democratic system of government, the North Vietnamese Army began to expand their territory by supporting the Vietcong, insurgents who were fighting against both foreign troops and South Vietnamese troops.
After World War I, Prime Minister Ho Chi Minh petitioned the U.S. to help remove the French from Vietnam to form a free nation; President Woodrow Wilson ignored the plea. Involvement by the U.S. was mild until after World War II, when the Korean War focused attention on the rise of communism in Indochina; since nuclear Russia were considered allies, the U.S. provisionally supported efforts for national freedom by Vietnam with supplies and military advice, but did not become directly involved until 1961, when President John F. Kennedy sent troops to South Vietnam to help train their forces.
Eventually, with escalating violence and a failure to hold positions against the guerrilla forces of the Vietcong, the U.S. sent hundreds of thousands of troops in to fight, and the war ultimately stretched for more than ten years. The initial goal of fighting back communism became lost in a push by President Lyndon B. Johnson to simply win without a farther-reaching goal, and it was not until President Richard Nixon's much-decried mission of "Vietnamization" that a goal of withdrawal began. Without U.S. support, South Vietnam quickly fell to communist control and remained under the power of North Vietnam, who were in turn supplied and controlled by Soviet Russia.