Final answer:
U.S. involvement in Vietnam shifted from financial and advisory support under Eisenhower and Kennedy to a full military escalation under Johnson, reflecting a significant change in American foreign policy and approach to Cold War containment.
Step-by-step explanation:
The involvement of the United States in Vietnam evolved significantly from the administrations of Eisenhower and Kennedy to that of Johnson. Under President Dwight D. Eisenhower, the U.S. provided substantial aid to France in its war against the Vietminh and later sent military advisors to South Vietnam following the Geneva Accords. This marked the beginning of U.S. involvement aimed at preventing the spread of communism, subscribing to the Domino Theory.
President John F. Kennedy continued support for South Vietnam and the Diem government. Kennedy's policy possibly hinted at a more covert and special operations-led involvement, as evidenced by actions such as the approval of secret bombing raids in Laos and assistance to the Hmong guerrillas.
With Lyndon B. Johnson (LBJ) assuming presidency after Kennedy's assassination, the stakes changed. Johnson dramatically escalated U.S. involvement, deploying more than half a million troops to Vietnam by 1965 and firmly committing to the fight against communism in the context of the Cold War. His administration saw a deepening military engagement, signaling a considerable shift from the advisory and indirect support roles played by previous administrations.
Ultimately, Johnson's decision to escalate the war overshadowed his domestic achievements, and the conflict in Vietnam became a defining and controversial aspect of his presidency.