From Wikipedia

Vietnam: The Last Battle

The Fall of Saigon was the capture of Saigon, the capital of South Vietnam, by the North Vietnamese army on April 30, 1975. The event marked the end of the Vietnam War and the start of a transition period leading to the formal reunification of Vietnam under communist rule.

North Vietnamese forces under the command of the Senior General Văn Tiến Dũng began their final attack on Saigon, which was commanded by General Nguyen Van Toan on April 29, with a heavy artillery bombardment. By the afternoon of the next day, North Vietnamese troops had occupied the important points within the city and raised their flag over the South Vietnamese presidential palace. South Vietnam capitulated shortly after. The city was renamed Ho Chi Minh City. The fall of the city was preceded by the evacuation of almost all the American civilian and military personnel in Saigon, along with tens of thousands of South Vietnamese civilians. The evacuation culminated in Operation Frequent Wind, which was the largest helicopter evacuation in history. In addition to the flight of refugees, the end of the war and institution of new rules by the communists contributed to a decline in the population of the city.