Escalation of U.S. Military Involvement in Vietnam

During the Vietnam War, the United States sought to prevent the spread of communism in Southeast Asia. This conflict, which lasted from the mid-1950s until the fall of Saigon in 1975, was marked by significant military engagement and profound social and political consequences at home and abroad. One policy that encapsulated the U.S. approach to the Vietnam War was the 'Domino Theory,' which suggested that the fall of one country to communism would lead to the fall of its neighbors. In light of this context, which of the following events escalated U.S. military involvement in Vietnam?