Sihanouk was condemned to death in absentia, an excessive move on the part of the new government that effectively ruled out any chance for com­promise over the next five years. Lon Nol gave communist Vietnamese forces an ultimatum to withdraw their forces within one week, which amounted to a virtual declaration of war, as no communists wanted to return to the homeland to face the Americans.

On 30 April 1970, US and South Vietnamese forces invaded Cambodia in an effort to flush out thousands of Viet Cong and North Vietnamese troops who were using Cambodian bases in their war to overthrow the South Vietnamese government. As a result of the invasion, the Vietnam­ese communists withdrew deeper into Cambodia, thus posing an even greater threat to the Lon Nol government. Cambodia's tiny army never stood a chance and within the space of a few months, Vietnamese forces and their Khmer Rouge allies controlled almost half the country. The ultimate humiliation came in July 1970 when the Vietnamese seized the temples of Angkor.

In 1969 the USA had begun a secret programme of bombing suspected communist base camps in Cambodia. For the next four years, until bomb­ing was halted by the US Congress in August 1973, huge areas of the eastern half of the country were carpet-bombed by US B-52s, killing what is believed to be many thousands of civilians and turning hundreds of thousands more into refugees. Some historians believe the bombing campaign may have killed as many as 250,000 Cambodians. Undoubt­edly, the bombing campaign helped the Khmer Rouge in their recruit­ment drive, as more and more peasants were losing family members to the aerial assaults. While the final, heaviest bombing in the first half of 1973 may have saved Phnom Penh from a premature fall, its ferocity also helped to harden the attitude of many Khmer Rouge cadres and may have contributed to the later brutality of the regime.

Savage fighting engulfed the country, bringing misery to millions of Cambodians; many fled rural areas for the relative safety of Phnom Penh and provincial capitals. Between 1970 and 1975 several hundred thousand people died in the fighting. During these years the Khmer Rouge came to play a dominant role in trying to overthrow the Lon Nol regime, strength­ened by the support of the Vietnamese, although the Khmer Rouge leader­ship would vehemently deny this from 1975 onwards.

The leadership of the Khmer Rouge, including Paris-educated Pol Pot and leng Sary, had fled into the countryside in the 1960s to escape the summary justice then being meted out to suspected leftists by Sihanouk's security forces. They consolidated control over the movement and began to move against opponents before they took Phnom Penh. Many of the Vietnamese-trained Cambodian communists who had been based in Hanoi since the 1954 Geneva Accords returned down the Ho Chi Minh Trail to join the Khmer Rouge in 1970. Many were dead by 1975, ex­ecuted on orders of the anti-Vietnamese Pol Pot faction. Likewise, many moderate Sihanouk supporters who had joined the Khmer Rouge as a show of loyalty to their fallen leader rather than a show of ideology to the radicals were victims of purges before the regime took power. This set a precedent for internal purges and mass executions that were to eventually bring the downfall of the Khmer Rouge.

It didn't take long for the Lon Nol government to become very un­popular as a result of unprecedented greed and corruption in its ranks. As the USA bankrolled the war, government and military personnel found lucrative means to make a fortune, such as inventing 'phantom soldiers' and pocketing their pay, or selling weapons to the enemy. Lon Nol was widely perceived as an ineffectual leader, obsessed by supersti­tion, fortune tellers and mystical crusades. This perception increased with his stroke in March 1971 and for the next four years his grip on reality seemed to weaken as his corrupt brother Lon Non's power grew.

Despite massive US military and economic aid, Lon Nol never succeeded in gaining the initiative against the Khmer Rouge, which pursued a strategy of rural attrition. Large parts of the countryside fell to the rebels and many provincial capitals were cut off from Phnom Penh. Lon Nol fled the coun­try in early April 1975, leaving Sirik Matak in charge, who refused evacu­ation to the end. 'I cannot alas leave in such a cowardly fashion...! have committed only one mistake, that of believing in you, the Americans' were the words Sirik Matak poignantly penned to US ambassador John Gunther Dean. On 17 April 1975 - two weeks before the fall of Saigon (now Ho Chi Minh City) - Phnom Penh surrendered to the Khmer Rouge.