The Khmer Rouge ordered the evacuation of Phnom Penh, emptying the city except for expatriates who took refuge in the French embassy until 30 April, when they were transported to Thailand.

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 the crucial Mekong supply route.

[1] On 27 January, seven vessels limped into Phnom Penh, the survivors of a 16-ship convoy that had come under attack during 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 Phú Mỹ, approximately 74 kilometres (46 mi) from Phnom Penh.

[1]: 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,[2]: 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 from U-Tapao Royal Thai Navy Airfield.

On 22 March, rockets hit two supply aircraft, forcing the American embassy to announce the following day a suspension of the airlift until the security situation improved.

Realizing that the Khmer Republic would soon collapse without supplies, the embassy reversed the suspension on 24 March and increased the number of aircraft available for the airlift.

The 3rd Division, located on Route 4 in the vicinity of Bek Chan (11°30′36″N 104°44′53″E / 11.51°N 104.748°E / 11.51; 104.748), some 10 kilometres (6.2 mi) west of Pochentong, was cut off from its own command post at Kompong Speu.

[3]: 161 The rapidly worsening situation of March was capped on the night of 1 April by the fall of Neak Luong, despite ferocious resistance and following a three-month siege.

This development opened the southern approach to the capital and freed up 6000 Khmer Rouge soldiers to join the forces besieging Phnom Penh.

[3]: 156–7 The last reserves of the high command, constituted by hastily taking the battalions of the former Provincial Guard, were rushed to the north, only to be completely dispersed by the Khmer Rouge after several hours of combat.

To the west, the troops of Brigadier General Norodom Chantaraingsey's 3rd Division, despite reinforcements, were unable to join with their own elements at Kompong Speu and retake the position at Toul Leap.

A computation error which caused FANK artillery fire to land on 3rd Division elements during the operation badly affected the unit's morale.

[3]: 157 On 11 April in Peking, the US Government requested the immediate return of Prince Norodom Sihanouk, the figurehead leader of the National United Front of Kampuchea (FUNK), to Phnom Penh.

Ambassador Dean invited the members of the government to be evacuated, but all refused except for acting President Saukham Khoy, who left without telling his fellow leaders.

[1]: 114  The evacuation came as a shock to many in the Khmer Republic leadership, because Phnom Penh and almost all provincial capitals (except those in the east occupied by the North Vietnamese) were in government hands, packed with millions of refugees.

Sak decided to make a last peace offer to Prince Sihanouk, transferring the Republic and its armed forces to him, but not surrendering to the Khmer Rouge.

[3]: 165 This Council made decisions including political and military measures, channeling the ever-increasing stream of refugees into schools, pagodas, their feeding, the reshuffling of the cabinet, reinforcing the troops in Phnom Penh by flying in a few battalions from different provinces through the Mean Chey airport and the formation of an Ad Hoc Committee chaired by Long Boret to prepare peace overtures for either Prince Sihanouk or the Khmer Rouge.

[3]: 166 [4]: 125 That afternoon Takhmau, the capital of the Kandal Province and 11 kilometres (6.8 mi) south of Phnom Penh, fell to the Khmer Rouge.

Pochentong and the dike running east–west to the north of Phnom Penh, both of which formed the last ring of defense around the capital, were overrun by Khmer Rouge assaults.

[3]: 167–8  Meanwhile the Khmer Rouge had occupied the east bank of the Mekong following the withdrawal of the Parachute Brigade, while General Dien Del’s 2nd Division held the Monivong Bridge.

After virtually expending their ordnance reserves, 97 KAF aircraft escaped from airbases and auxiliary airfields throughout Cambodia, with a small number of civilian dependents on board to safe havens in neighbouring Thailand.

After 06:00, the Minister of Information, Thong Lim Huong, brought a cable that just arrived from Peking advising that the peace appeal had been rejected by Sihanouk.

[9][2]: 365 [4]: 136–7 Several hours later the Khmer Rouge began entering central Phnom Penh from all directions and stationed themselves at the major crossroads where they disarmed FANK soldiers and collected weapons.

[10]: 192–3 On the morning of 18 April, Sak and the remaining members of the Khmer Republic Government and assorted military personnel boarded a KAF C-123 and flew to U-Tapao and into exile.

Among those evicted was Prince Sisowath Sirik Matak, one of those responsible for the removal of Sihanouk from power in 1970 and who had been branded one of the seven original traitors marked for execution by the FUNK.

Supporters of the Khmer Republic and the intelligentsia were killed, while the former urban population was used as forced labor in the countryside, many dying from physical abuse and malnutrition.

[10]: 195  This led to a series of clashes between Vietnam and Cambodia on several islands in May 1975 and the seizure of foreign ships by the Khmer Rouge, which triggered the Mayaguez incident.