Final answer:
President Lyndon B. Johnson escalated U.S. involvement in the Vietnam War primarily due to his belief in the domino theory, which suggested that a Communist Vietnam could lead to subsequent Communist takeovers in neighboring countries, posing a threat to U.S. interests.
Step-by-step explanation:
President Lyndon B. Johnson entered the Vietnam War because he believed in the domino theory. This was the idea that if one country in a region came under the influence of Communism, then the surrounding countries would follow in a domino effect, potentially threatening U.S. interests and allies. Johnson's administration feared that the fall of South Vietnam to Communist forces would lead to further spread of Communism throughout Southeast Asia. As such, the U.S. effort in Vietnam was intensified to support South Vietnam against the Communist North and prevent the domino effect. This led to a significant escalation of American military presence by 1965 with over half a million U.S. soldiers on the ground in Vietnam.
The domino theory informed U.S. foreign policy during the Cold War and was a key motivator in Johnson's decision to increase U.S. involvement in Vietnam following the instability after President Diem's death and subsequent succession of weak governments. Despite widespread domestic opposition and protest, and the controversial nature of supporting the non-Communist dictatorship in South Vietnam, Johnson remained committed to the war effort, partly driven by the fear of being perceived as soft on Communism.