On New Year's Day of January 1975 the Khmer Rouge launched what it hoped was the final assault on Phnom Penh. The Cambodian capital was now swollen with over two million refugees. Access to food supplies in the countryside was completely cut off, and Phnom Penh starved slowly as a stream of US airlifts unsuccessfully attempted to feed the entire city with less than 600 metric tons of food per day. Despite a brave fight, Lon Nol's troops quickly fell apart from lack of supplies, lack of support, and lack of leadership. The now-fanatical Khmer Rouge, strengthened by a steady stream of supplies from Hanoi and emboldened by surviving years of sustained US bombardment, made their push into the Phnom Penh suburbs. By the end of March it was clear there was no way of stopping the Khmer Rouge siege.
On April 1, a weeping Lon Nol, crippled by nervous breakdowns and a series of minor strokes, fled Phnom Penh for Hawaii with his family and entourage while Prince Sirik Matak and other Lon Nol supporters remained behind in the hopes of organizing a last-minute peace talks. The Khmer Rouge rejected the talks and pressed further into the capital. US Ambassador to Cambodia John Gunther Dean quickly made plans to evacuate US embassy staff and their families along with key Cambodian government officials, including Sirik Matak, Lon Nol's brother Lon Non, and acting prime minister Long Boret. All three declined the offer. In the hours leading up to the evacuation Sirik Matak responded to Dean's invitation:
Dear Excellency and friend,
I thank you very sincerely for your letter and for your offer to transport me towards freedom. I cannot, alas, leave in such a cowardly fashion.
As for you and in particular for your great country, I never believed for a moment that you would have this sentiment of abandoning a people which has chosen liberty. You have refused us your protection and we can do nothing about it. You leave us and it is my wish that you and your country will find happiness under the sky.
But mark it well that, if I shall die here on the spot and in my country that I love, it is too bad because we are all born and must die one day. I have only committed the mistake of believing in you, the Americans.
Please accept, Excellency, my dear friend, my faithful and friendly sentiments. Sirik Matak.
Prince Sisowath Sirik Matak, former contender for the Cambodian throne and co-conspirator in the Lon Nol coup, would be executed by the Khmer Rouge two weeks later, along with Long Boret, Lon Non, and the other remaining members of the Lon Nol government.
On the morning of April 12, Ambassador John Gunther Dean and the US embassy staff boarded a series of US transport helicopters to evacuate to a navy ship waiting in the Gulf of Thailand. Khmer children observing the evacuation waved to the Americans, calling out "OK, bye-bye, OK, bye-bye" to the departing embassy staff. As the helicopters departed Phnom Penh, the Khmer Rouge shelled the evacuation zone, firing mortars into the crowd watching the departure. The civil war was coming to an end.
Five days later, on April 17, 1975, Khmer Rouge forced marched unopposed into central Phnom Penh. At first the residents of the city celebrated - the siege was over, there would be no more fighting. But within hours, the joy would turn to horror as the Khmer Rouge began to implement their barbarous plan for a utopian communist society. April 17, 1975 was Day Zero for the new Cambodia - two thousand years of Khmer history were now meaningless.