The main cause of the Vietnam War was the conflict between North Vietnam, which was communist, and South Vietnam, backed by the United States and other anti-communist allies. The U.S. aimed to contain the spread of communism in Southeast Asia, influenced by the domino theory, which suggested that if one country fell to communism, others would follow. This led to increased military involvement and support for the South Vietnamese government's fight against the Viet Cong and North Vietnamese forces, ultimately escalating into a prolonged and controversial conflict that lasted from the mid-1950s until 1975.