Strangulation of Phnom Penh
At the beginning of 1975 the Khmer Republic, a United States-supported military government, controlled only the Phnom Penh area and a string of towns along the Mekong River that provided the crucial supply route for food and munitions coming upriver from South Vietnam. As part of their 1975 dry season offensive, rather than renewing their frontal attacks on Phnom Penh, the Khmer Rouge set out to cut off the crucial Mekong supply route. On 12 January 1975 the Khmer Rouge attacked Neak Luong, a key Khmer National Armed Forces (FANK) defensive outpost on the Mekong. On 27 January, seven vessels limped into Phnom Penh, the survivors of a 16-ship convoy that had come under attack over the 100 kilometres (62 mi) journey from the South Vietnamese border. On 3 February a convoy heading downriver hit naval mines laid by the Khmer Rouge at Phu My approximately 74 kilometres (46 mi) from Phnom Penh. The FANK naval branch, the Khmer National Navy (MNK), had mine-sweeping capability, but due to the Khmer Rouge control of the riverbanks mine-sweeping was impossible or at best extremely costly. The MNK had lost a quarter of its ships, and 70 percent of its sailors had been killed or wounded.
By 17 February, the Khmer Republic abandoned attempts to reopen the Mekong supply line. In future, all supplies for Phnom Penh would have to come in by air to Pochentong Airport.:105 The United States quickly mobilised an airlift of food, fuel and ammunition into Phnom Penh, but as US support for the Khmer Republic was limited by the Case–Church Amendment,:347 BirdAir, a company under contract to the US Government, controlled the airlift with a mixed fleet of C-130 and DC-8 planes, flying 20 times a day into Pochentong.:105
On 5 March, Khmer Rouge artillery at Toul Leap, north-west of Phnom Penh, shelled Pochentong Airport, but FANK troops recaptured Toul Leap on 15 March and ended the shelling. Khmer Rouge forces continued to close in on the north and west of the city and were soon able to fire on Pochentong again. On 22 March rockets hit two supply aircraft, forcing the US Embassy to announce on 23 March a suspension of the airlift until the security situation improved. The Embassy, realizing that the Khmer Republic would soon collapse without supplies, reversed the suspension on 24 March and increased the number of aircraft available for the airlift. On 1 April the Khmer Rouge overran Neak Luong and Ban-am, the last remaining FANK positions on the Mekong. The Communists could now concentrate all their forces on Phnom Penh.:105 Premier Lon Nol resigned that day and went into exile; the final collapse of the Khmer Republic was imminent.:358