The main reason for U.S. involvement in the Vietnam War was to prevent the spread of communism in Southeast Asia as part of the broader strategy of containment during the Cold War. After the division of Vietnam into North and South, the U.S. supported the anti-communist government in the South, fearing that a communist victory would lead to a domino effect throughout the region. The conflict escalated in the 1960s, with increased military involvement and a commitment to protect the South Vietnamese government. This war became highly controversial in the U.S., sparking significant protests and debates over foreign policy and military engagement.