The main cause of the Vietnam War's escalation was the U.S. commitment to prevent the spread of communism in Southeast Asia. Following the Gulf of Tonkin Incident in 1964, American involvement significantly increased through military aid and ground troops, driven by the Domino Theory. This theory posited that if one country fell to communism, neighboring countries would follow. The subsequent policy of escalation resulted in a political and social backlash, leading to widespread protests in the U.S. and contributing to a growing anti-war sentiment among various segments of the population.