Sihanouk was away on a trip to Moscow and Beijing when General Lon Nol launched a successful coup d'état. On the morning of March 18, 1970, the National Assembly was hastily convened, and voted unanimously to depose Sihanouk as head of state. Lon Nol, who had been serving as prime minister, was granted emergency powers. Sirik Matak, an ultraconservative royal prince who in 1941 had been passed over by the French in favor of his cousin Norodom Sihanouk as king, retained his post as deputy prime minister. The new government emphasized that the transfer of power had been totally legal and constitutional, and it received the recognition of most foreign governments.

Most middle-class and educated Khmers in Phnom Penh had grown weary of Sihanouk and apparently welcomed the change of government. But he was still popular in the villages. Days after the coup, the prince, now in Beijing, broadcast an appeal to the people to resist the usurpers. Demonstrations and riots occurred throughout the country. In one incident on March 29, an estimated 40,000 peasants began a march on the capital to demand Sihanouk's reinstatement. They were dispersed, with many casualties, by contingents of the armed forces and the Khmer Serei.

From Beijing, Sihanouk proclaimed his intention to create a National United Front of Kampuchea (Front Uni National du Kampuchéa--FUNK--see Appendix B). In the prince's words, this front would embrace "all Khmer both inside and outside the country-- including the faithful, religious people, military men, civilians, and men and women who cherish the ideals of independence, democracy, neutrality, progressivism, socialism, Buddhism, nationalism, territorial integrity, and anti-imperialism." A coalition, brokered by the Chinese, was hastily formed between the prince and the KCP. On May 5, 1970, the actual establishment of FUNK and of the Royal Government of National Union of Kampuchea (Gouvernement Royal d'Union Nationale du Kampuchéa--GRUNK--see Appendix B), were announced. Sihanouk assumed the post of GRUNK head of state, appointing Penn Nouth, one of his most loyal supporters, as prime minister. Khieu Samphan was designated deputy prime minister, minister of defense, and commander in chief of the GRUNK armed forces (though actual military operations were directed by Pol Pot). Hu Nim became minister of information, and Hou Yuon assumed multiple responsibilities as minister of interior, communal reforms, and cooperatives. GRUNK claimed that it was not a government-in-exile because Khieu Samphan and the insurgents remained inside Cambodia.

For Sihanouk and the KCP, this was an extremely useful marriage of convenience. Peasants, motivated by loyalty to the monarchy, rallied to the FUNK cause. The appeal of the Sihanouk-KCP coalition grew immensely after October 9, 1970, when Lon Nol abolished the monarchy and redesignated Cambodia as the Khmer Republic. The concept of a republic was not popular with most villagers, who had grown up with the idea that something was seriously awry in a Cambodia without a monarch.

GRUNK operated on two tiers. Sihanouk and his loyalists remained in Beijing, although the prince did make a visit to the "liberated areas" of Cambodia, including Angkor Wat, in March 1973. The KCP commanded the insurgency within the country. Gradually, the prince was deprived of everything but a passive, figurehead role in the coalition. The KCP told people inside Cambodia that expressions of support for Sihanouk would result in their liquidation, and when the prince appeared in public overseas to publicize the GRUNK cause, he was treated with almost open contempt by Ieng Sary and Khieu Samphan. In June 1973, the prince told the Italian journalist Oriana Fallaci that when "they [the Khmer Rouge] no longer need me, they will spit me out like a cherry pit!" By the end of that year, Sihanouk loyalists had been purged from all of GRUNK's ministries.

Data as of December 1987