The primary cause of the Vietnam War was the conflict between communist North Vietnam and U.S.-supported South Vietnam, rooted in the broader context of the Cold War. The U.S. aimed to curb the spread of communism in Southeast Asia, viewing it as a critical front in the ideological battle against Soviet influence. The U.S. believed in the "domino theory," which suggested that the fall of one nation to communism could trigger similar events in neighboring nations. This led to escalating American involvement in the 1960s, ultimately resulting in significant military engagement and widespread protests and opposition at home.