It is called the "Ngày mất nước" (Day we Lost the Country), "Tháng Tư Đen" (Black April), The rapidity with which the South Vietnamese position collapsed in 1975 was surprising to most American and South Vietnamese observers, and probably to the North Vietnamese and their allies as well. These predictions proved to be grievously in error.
Even as that memo was being released, General Dũng was preparing a major offensive in the Central Highlands of Vietnam, which began on 10 March and led to the capture of Buôn Ma Thuột.
The ARVN began a disorderly and costly retreat, hoping to redeploy its forces and hold the southern part of South Vietnam, perhaps an enclave south of the 13th parallel.
Along the way, disorderly South Vietnamese retreats and the flight of refugees—there were more than 300,000 in Đà Nẵng—damaged South Vietnamese prospects for a turnaround.
After the loss of Đà Nẵng, those prospects had already been dismissed as nonexistent by American Central Intelligence Agency officers in Vietnam, who believed that nothing short of B-52 strikes against Hanoi could possibly stop the North Vietnamese.
Meanwhile, South Vietnam failed to garner any significant increase in military aid from the United States, snuffing out President Nguyễn Văn Thiệu’s hopes for renewed American support.
On April 9, PAVN forces reached Xuân Lộc, the last line of defense before Saigon, where the ARVN 18th Division made a last stand and held the city through fierce fighting for several days.
The PAVN finally overran Xuân Lộc on April 20 despite heavy losses, and on April 21 President Thiệu resigned in a tearful televised announcement in which he denounced the United States for failing to come to the aid of the South.
opened the way for PAVN to encircle Saigon, and they soon did so, moving 100,000 troops in position around the city by April 27.
With the ARVN having few defenders, the fate of the city was effectively sealed.
The ARVN III Corps commander, General Toan, had organized five centers of resistance to defend the city.
was the capture of Saigon, the capital of South Vietnam, by the People’s Army of Vietnam and the National Liberation Front of South Vietnam (also known as the Việt Cộng) on April 30, 1975.