What strategy did the United States adopt in Vietnam based on "domino theory"?

The United States committed millions of troops to the conflict in Vietnam because it feared that surrounding countries would fall to communism if Vietnam did.

The United States withdrew its troops from Vietnam after a strong North Vietnamese offensive during the Vietnamese new year celebration.

The United States negotiated the peaceful transition of power from colonial French administrators to popularly elected Vietnamese leaders.

The United States and the Soviet Union negotiated the division of Vietnam into separate spheres of influence that eventually became North and South Vietnam.