The Vietnam War was primarily caused by America's fear of the spread of communism in Southeast Asia, influenced by events from the Cold War, notably the Korean War. The US believed in the 'domino theory,' which suggested that if one nation fell to communism, nearby countries would follow suit, leading to a request for assistance from South Vietnam. Ultimately, despite the US’s involvement, it failed to prevent the collapse of South Vietnam as seen in the stark contrast with South Korea's success post-conflict.