Why did the United States support South Vietnam after the Geneva Accords?

Following the 1954 Geneva Accords, the United States chose not to be bound by the agreements and instead committed to building an anticommunist state in South Vietnam. This support was driven by several key factors:

• Containment Policy: The U.S. sought to halt the spread of communism in Asia, viewing the defense of South Vietnam as essential to maintaining global credibility and prestige against the influence of the Soviet Union and China.1
• Domino Theory: U.S. policymakers were guided by the domino theory, which posited that if one noncommunist state fell to communism, it would inevitably lead to the collapse of neighboring governments.21
• State Building: The U.S. provided extensive military and economic aid to the government of Ngo Dinh Diem, training his army and using CIA operatives to stabilize his rule against internal opposition and communist influence.31

By 1956, the U.S. further supported South Vietnam’s refusal to participate in nationwide reunification elections, fearing that a communist victory would undermine regional security.4