The last event that occurred in the Vietnam War from the options you provided is the Publication of the Pentagon Papers. Here's a brief timeline of the events listed:
- Battle of Dien Bien Phu - This occurred in 1954 and marked the end of French colonial rule in Indochina.
- Gulf of Tonkin Incident - This happened in August 1964 and led to increased U.S. involvement in Vietnam.
- My Lai Massacre - This occurred in March 1968 and was a significant event during the war, revealing the brutality of U.S. actions in Vietnam.
- Publication of Pentagon Papers - These were published in 1971 and revealed undisclosed information about the U.S. government’s involvement in Vietnam, leading to public outcry and debates over the war.
Therefore, the event that happened last is the Publication of the Pentagon Papers.