The capture of the city was preceded by Operation Frequent Wind, the evacuation of almost all the American civilian and military personnel in Saigon, along with tens of thousands of South Vietnamese civilians who had been associated with the southern regime. A few Americans chose not to be evacuated. US ground combat units had left South Vietnam more than two years prior to the fall of Saigon and were not available to assist with either the defense of Saigon or the evacuation. The evacuation was the largest helicopter evacuation in history. In addition to the flight of refugees, the end of the war and the institution of new rules by the communists contributed to a decline in the city's population.