America's increase of participation in the Vietnam War was mainly caused by the long-running causes of the cold war, which were fear of communism and containment.
The Vietnam War really started in 1945 with the North Vietnamese Declaring independence. Vietnam was a French colony before the Second World War and, with the liberation of German controlled France; they decided to try to reclaim their imperial state for the French empire. The leader of the Vietnam (Ho Chi Minh) had been shown possibility of support from America during the fighting against the Japanese, so he thought they would help him in the war with France. Unfortunately for him it was discovered that Ho Chi Minh was "sympathetic" to communism and the Americans had announced their efforts to try to contain it. The French also invoked the Truman Doctrine as, technically, they were fighting against a communist aggressor. America sent supplies, equipment and money to Vietnam for the French soldiers.
The war with France ended in 1954 with Ho chi Minh delivering a crushing defeat to France at Dien Bien Phu. The USSR and China recognized Vietnam's independence and this meant the rest of the world was practically made to recognize it was independent, though many countries just ignored what was happening.
After the defeat of France a convention was held in Geneva where it was decided that Vietnam, like Korea was to be split in two with the communists getting the north and the democrats getting the south with elections in 1956 for the people of Vietnam to decide what party they wanted in power. America believed in the "Domino Theory." This suggested that, because of China, Japan, the USSR and North Korea becoming communist they believed that if Vietnam fell to communism that the neighbouring countries of Laos and Cambodia...