The Vietnam War stemmed primarily from the consequences of the Cold War. The United States continued to hold the belief that communism was threatening to American society, which made them naturally clash with the main communist power in the world at the time: The Soviet Union. These two countries had opposing views on how the world should be organized after World War 2. Both sides understood that they could not afford to actually be involved in fighting against each other as it would financially deplete both countries. Since the U.S. got directly involved in the fighting of the Vietnam War, the Soviets were forced to step back from the fighting. Despite this, the Soviets continued to support the Communist cause by financing China and North Vietnam in the fight against the Americans and French.
In the early 1900's, Northern Vietnam was controlled by the French Empire as a territory. During World War 2, the Japanese took over the area but soon retreated following their surrender at the hands of the U.S. Vietnam was now a free state and established its own government led by Ho Chi Minh. At the end of World War 2, the Allied Powers established that South Vietnam would be returned to the French while the North would be given to the non-Communist Chinese. These Nationalists from China treated the North Vietnamese very harshly and these natives longed to have Ho Chi Minh return to power. His communist party, called the Viet Minh, took back control of the region in 1946 when the Chinese pulled out of North Vietnam.
It was not long until another threat to the Viet Minh party arose, as the French expressed interest in repossessing North Vietnam in October, 1946. President Harry S. Truman was in office during this time, and he believed that it was vital for the US to strengthen its alliances with other countries. He thought it vital for the U.S.to assure that all countries opposing the USSR and Communism would be allies of the United States. Truman believed that this was the only way to amass a large enough global menace that could finally take down the Soviets and Communism for good. This belief resulted in a legislation passed by Congress called the Truman Doctrine, which promised millions of dollars of military aid for weapons and training. This doctrine also resulted in the U.S. joining the fighting of the Vietnam War, which officially started in November, 1946.