The Khmer Rouge initiated their dry-season offensive to capture the beleaguered Cambodian capital on January 1, 1975. Their troops controlled the banks of the Mekong River, and they were able to rig ingenious mines to sink convoys bringing relief supplies of food, fuel, and ammunition to the slowly starving city. After the river was effectively blocked in early February, the United States began airlifts of supplies. This was extremely risky because of Khmer Rouge rockets. The communists also fired rockets and shells into the city, causing many civilian deaths. Doomed units of republican soldiers dug in around the capital many of them had run out of ammunition, and they were overrun as the Khmer Rouge advanced. American observers, who generally had little esteem for FANK officer corps, were impressed by the determination of the Khmer enlisted men to fight to the end.

On April 1, 1975, President Lon Nol resigned and left the country. His exit was prompted by fear of certain death if he fell into Khmer Rouge hands. The communists had included him among "seven traitors" who were marked for execution. (The others were non-communist, nationalist leaders Sirik Matak, Son Ngoc Thanh, In Tam, Prime Minister Long Boret, Cheng Heng, who became head of state after Sihanouk's ouster, and Sosthene Fernandez, the FANK commander in chief). Saukham Khoy became acting president of a government that had less than three weeks to live. Last-minute efforts on the part of the United States to arrange a peace agreement involving Sihanouk ended in failure. On April 12, United States embassy personnel were evacuated by helicopter. The ambassador, John Gunther Dean, invited high officials of the Khmer Republic to join them. But Sirik Matak, Long Boret, Lon Non (Lon Nol's brother), and most members of Lon Nol's cabinet declined. They chose to share the fate of their people. All were executed soon after Khmer Rouge units entered Phnom Penh on April 17, 1975.

Data as of December 1987