Japan conquered IndoChina (Vietnam, Cambodia, Laos) during WWII. After the war, the French went back to retake their colonial territory, but communist fighters, the Viet Minh, were trying to also take control. The First Indochina War broke out. (The Vietnam War is also known as the Second Indochina War)
So, the US entered the war because: the US signed the Mutual Defense Assistance Act with France in 1950 and with growing concerns about the spread of communism in Asia after the Maoist revolution in China installed the CCP in 1949, the start of the Korean War in 1950, the US decided to start to provide military assistance to France in the war that was viewed as a proxy war against China and the USSR. The French finally surrendered land north of the 17 parallel and would leave Vietnam.
After the war, the Geneva Conference on July 21, 1954 made a provisional division of Vietnam at the 17th parallel, pending unification on the basis of internationally supervised free elections. Control of the north was given to the Viet Minh under Ho Chi Minh, and the south continued under Emperor Bảo Đại. A year later, Bảo Đại would be deposed by his prime minister, Ngô Đình Diệm, creating the Republic of Vietnam. Soon an insurgency backed by the North developed against Diệm's government. The conflict gradually escalated into the Vietnam War.