Final answer:
The U.S. entered the Vietnam War to support South Vietnam's non-Communist government as part of its Cold War strategy to prevent the spread of Communism.
Step-by-step explanation:
The United States got involved in the Vietnam War primarily to support a democratic government in South Vietnam against the Communist forces from North Vietnam. During the Cold War, the U.S. sought to contain the spread of Communism as part of its global strategic objectives. The U.S. involvement escalated under President Lyndon Johnson, who did not want to see Vietnam and its neighboring countries fall to Communism, fearing a domino effect in Southeast Asia. This intervention is often seen as a manifestation of the conflicting ideologies between democracy and Communism that defined the era.