<P> The U.S. was motivated by the desire to buy time for its withdrawal from Southeast Asia, to protect its ally in South Vietnam, and to prevent the spread of communism to Cambodia . American and both South and North Vietnamese forces directly participated (at one time or another) in the fighting . The U.S. assisted the central government with massive U.S. aerial bombing campaigns and direct material and financial aid . </P> <P> After five years of savage fighting, the Republican government was defeated on 17 April 1975 when the victorious Khmer Rouge proclaimed the establishment of Democratic Kampuchea . The war caused a refugee crisis in Cambodia with two million people--more than 25 percent of the population--displaced from rural areas into the cities, especially Phnom Penh which grew from about 600,000 in 1970 to an estimated population of nearly 2 million by 1975 . The Cambodian government estimated that more than 20 percent of the property in the country had been destroyed during the war . In total, 240,000--800,000 people were killed as a result of the war . </P> <P> The conflict was part of the Second Indochina War (1959--1975) which also consumed the neighboring Kingdom of Laos, South Vietnam, and North Vietnam individually referred to as the Laotian Civil War and the Vietnam War respectively . The Cambodian civil war led to the Cambodian Genocide, one of the bloodiest in history . </P> <P> During the early - to - mid-1960s, Prince Norodom Sihanouk's policies had protected his nation from the turmoil that engulfed Laos and South Vietnam . Neither the People's Republic of China (PRC) nor North Vietnam disputed Sihanouk's claim to represent "progressive" political policies and the leadership of the prince's domestic leftist opposition, the Pracheachon Party, had been integrated into the government . On 3 May 1965, Sihanouk broke diplomatic relations with the U.S., ended the flow of American aid, and turned to the PRC and the Soviet Union for economic and military assistance . </P>

What two countries did the u.s. expand their fighting to in 1970