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). 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), 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.

The 1970 coup d'état that toppled Sihanouk dragged Cambodia into the vortex of a wider war. The escalating conflict pitted government troops, now renamed the Khmer National Armed Forces (Forces Armées Nationales Khmères--FANK), initially against the North Vietnamese and the Viet Cong, and subsequently against the old RAK, now revitalized and renamed the Cambodian People's National Liberation Armed Forces (CPNLAF).

As combat operations quickly disclosed, the two sides were mismatched. The inequality lay not so much in sheer numbers. Thousands of young urban Cambodians flocked to join FANK in the months following the coup and, throughout its five-year life, the republican government forces held a numerical edge over their opponents, the padded payrolls and the phantom units reported in the press notwithstanding. Instead, FANK was outclassed in training and leadership. With the surge of recruits, the government forces expanded beyond their capacity to absorb the new inductees. Later, given the press of tactical operations and the need to replace combat casualties, there was insufficient time to impart needed skills to individuals or to units, and lack of training remained the bane of FANK's existence until its collapse. While individual soldiers and some government units fought bravely, their leaders-- with notable exceptions--were both corrupt and incompetent. Arrayed against an armed force of such limited capability was arguably the best light infantry in the world at the time--the North Vietnamese and the Viet Cong. And when there forces were supplanted, it was by the tough, rigidly indoctrinated peasant army of the CPNLAF with its core of Khmer Rouge leaders.

With the fall of Sihanouk, the North Vietnamese and the Viet Cong became alarmed at the prospect of a pro-Western regime that might allow the United States to establish a military presence on their western flank. To prevent this from happening, they began transferring their military installations away from the border area to locations deeper within Cambodian territory. A new command center was established at the city of Kracheh (Kratié). On April 29, 1970, South Vietnamese and United States units unleashed a multi-pronged offensive into Cambodia to destroy the Central Office for South Vietnam (COSVN), the headquarters for North Vietnamese and Viet Cong combat operations in South Vietnam. Extensive logistical installations and large amounts of supplies were found and destroyed, but as reporting from the United States MACV subsequently disclosed, still larger amounts of material already had been moved deeper into Cambodia.

The North Vietnamese army turned on the republican government forces, and by June 1970, three months after the coup, they and the CPNLAF had swept FANK from the entire northeastern third of the country. After defeating the government forces, they turned newly won territories over to the local insurgents. The Khmer Rouge also established "liberated areas" in the south and the southwestern parts of the country, where they operated independently of the Vietnamese. The KCP's debt to the North Vietnamese after March 1970 was one that Pol Pot was loath to acknowledge; however, it is clear that without North Vietnamese and Viet Cong assistance, the revolutionary struggle would have dragged on much longer than it did.

United States bombing of enemy troop dispositions in Cambodia-- particularly in the summer of 1973, when intense aerial bombardment (known as Arclight) was used to halt a Khmer Rouge assault on Phnom Penh--bought time for the Lon Nol government, but did not stem the momentum of the communist forces. United States official documents give a figure of 79,959 sorties by B-52 and F-111 aircraft over the country, during which a total of 539,129 tons of ordnance were dropped, about 350 percent of the tonnage (153,000 tons) dropped on Japan during World War II. Many of the bombs that fell in Cambodia struck relatively uninhabited mountain or forest regions; however, as declassified United States Air Force maps show, others fell over some of the most densely inhabited areas of the country, such as Siemreab Province, Kampong Chhnang Province, and the countryside around Phnom Penh. Deaths from the bombing are extremely difficult to estimate, and figures range from a low of 30,000 to a high of 500,000. Whatever the real extent of the casualties, the Arclight missions over Cambodia, which were halted in August 15, 1973, by the United States Congress, delivered shattering blows to the structure of life in many of the country's villages, and, according to some critics, drove the Cambodian people into the arms of the Khmer Rouge.

The bombing was by far the most controversial aspect of the United States presence in Cambodia. In his book Sideshow, William Shawcross provides a vivid image of the hellish conditions, especially in the months of January to August 1973, when the Arclight sorties were most intense. He claims that the bombing contributed to the forging of a brutal and singlemindedly fanatical Khmer Rouge movement. However, his arguments have been disputed by several United States officials--including the former ambassador to Cambodia, Emory C. Swank, and the former Air Force commander in Thailand, General John W. Vogt--in an appendix to the second volume of the memoirs of then Secretary of State, Henry Kissinger.

From the Khmer Rouge perspective, however, the severity of the bombings was matched by the treachery of the North Vietnamese. The Cambodian communists had refused to take part in the Paris peace talks. When North Vietnam and the United States signed the Paris Peace Accords on January 27, 1973, bombing missions over Vietnam and Laos were terminated. The fighter bombers and other aircraft thus released were diverted to strike Khmer Rouge positions in Cambodia.

One of the earliest accounts of life under the Khmer Rouge was written in 1973 by a school administrator, Ith Sarin, who had joined the movement after becoming disillusioned with Lon Nol and the Khmer Republic, then rose to the status of candidate member of the KCP, but left the party and returned to Phnom Penh after nine months in the underground. His work, Regrets for the Khmer Soul (in Khmer, Sranaoh Pralung Khmer), it revealed the secrecy with which the Khmer Rouge concealed the existence of the communist party, which they referred to by the sinister term Angkar Loeu (High Organization), or simply, Angkar. The KCP Central Committee was referred to as the Kena Mocchhim (or Committee Machine, mocchhim being derived from the Western term, "machine").

Territories under Angkar control were well organized. Ith Sarin described a five-level hierarchy of Angkar-controlled bodies reaching from the six areas, or phumphaek into which the country was divided down to the hamlet, or phum level. The Angkar imposed a grim regime in which hatred for Lon Nol, the Americans, and, at times, the North Vietnamese "allies" was assiduously cultivated. Expressions of support for Sihanouk were firmly discouraged and people were encouraged to spy on each other. Discipline was unremittingly harsh. Ith Sarin concluded from his experience that the great majority of the people did not like the Angkar and the collective way of life it imposed, that they despaired that Sihanouk would ever return to power, and that they would support the Khmer Republic if it carried out genuine reforms. Oddly, Lon Nol's security forces banned the book for a time on the grounds that it was "pro-communist." Although this was not true, it did provide a foretaste of what the entire Cambodian population would endure after April 1975.

Disturbing stories of Khmer Rouge atrocities began to surface as the communists prepared to deal the coup de grace to the Khmer Republic. In March 1974, they captured the old capital city of Odongk north of Phnom Penh, destroyed it, dispersed its 20,000 inhabitants into the countryside, and executed the teachers and civil servants. The same year, they brutally murdered sixty people, including women and children, in a small village called Sar Sarsdam in Siemreab Province. A similar incident was reported at Ang Snuol, a town west of the capital. Other instances of what one observer, Donald Kirk, described as a "sweeping, almost cosmic policy" of indiscriminate terror, were recounted by refugees who fled to Phnom Penh or across the Thai border. Kirk contrasted this behavior with the Viet Cong's use of "a modicum of care and precision" in applying terror in South Vietnam (for instance, assassination of landlords or of South Vietnamese officials). Atrocity stories, however, were considered to be anticommunist propaganda by many, if not most, Western journalists and other observers; nevertheless, Phnom Penh's population swelled to as many as 2.5 million people as terrified refugees sought to escape not only the United States bombing and the ground fighting, but the harshness of life under the Angkar.

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.