Final answer:
Eisenhower's belief in the domino theory led to U.S. efforts to contain communism by supporting South Vietnam after the French withdrawal, providing economic and military aid, and sending military advisors to prevent a domino effect of communist takeovers in Southeast Asia. (option A)
Step-by-step explanation:
President Dwight D. Eisenhower's belief in the domino theory shaped U.S. policy toward Vietnam significantly. Eisenhower saw Vietnam as a 'key domino' in the spread of communism in Southeast Asia. After the French defeat at Dien Bien Phu and their subsequent withdrawal, the U.S. increased its support for South Vietnam to prevent a communist takeover, fearing that if Vietnam fell, neighboring countries would follow. This marked a continuation and escalation of the policy of containment, supporting non-communist regimes to prevent further communist expansion, even when such regimes were not entirely aligned with U.S. values.
The Eisenhower administration aided the French in their struggle against the Viet Minh and, following the French departure refused to support the unification of Vietnam under communist Chi Minh. Instead, the U.S. provided economic and military aid to prop up the South Vietnamese government and sent military advisors, thus avoiding direct military engagement while still attempting to contain the spread of communism as part of its Cold War strategy.