The murder of the president of South Vietnam, Ngo Dinh Diem, and his powerful brother and adviser, on November 2, 1963, was a major turning point in the war in Vietnam. Up until the deaths of the Ngo brothers, the United States had been advising the government of South Vietnam in its war against the Viet Cong and their benefactors, the government of North Vietnam. At the time, the United States had 16,000 troops in South Vietnam training the ARVN forces and even going so far as to accompany them on helicopter-borne raids deep into enemy territory. American casualties were beginning to mount, and images of the dead were being broadcast on stateside network television.
In the wake of the assassinations, American policy toward the war in Vietnam changed dramatically. The murder of President John F. Kennedy almost three weeks later placed a new head of state in the White House. Lyndon B. Johnson carried on his predecessor’s Vietnam. The credibility gap came into light and it was a term that came into wide use with journalism, political and public discourse in the United States. At the time, it was most frequently used to describe public skepticism about the Lyndon B. Johnson administration's statements and policies on the Vietnam War.