In February 1973, during the Cambodian Civil War, Phnom Penh citizens were indiscriminately killed day after day in rocket attacks on the capital Phnom Penh by the Khmer Rouge. During the Cambodian Civil War, the U.S. military bombing of rural areas caused the rural population to flow into the cities as refugees, increasing the population of the capital Phnom Penh to more than about 2 million people. The aerial bombardment by the U.S. military reached three times the total amount dropped on Japan in the Pacific War. Hundreds of thousands of farmers and agricultural infrastructure were killed, and the entire Cambodian land was turned into scorched earth. In 1973, Phnom Penh was besieged by the Khmer Rouge and the U.S. Air Force was launched, and from 1972 to 1975, the civil war raged mainly along the Khmer National Front line of communication, north and south of the capital city of Phnom Penh.
In the Cambodian Civil War, the Kingdom of Cambodia was overthrown in a coup d'état in March 1970 and a military junta established the Khmer Republic, which continued until 1993 when elections to the Cambodian National Assembly brought a democratic government to power. In January 1972, the U.S. military sent part of its South Vietnamese contingent to invade Cambodia on behalf of the Lon Nol military regime. In January 1972, the U.S. invaded Cambodia with part of its South Vietnamese contingent on behalf of the Lon Nol military regime, directly intervening in the Cambodian civil war. The Vietnam War expanded into the Indochina War. In October 1972, Lon Nol declared a military dictatorship, and in March 1973, he promulgated a new constitution that gave the president dictatorial powers.
The Khmer Rouge, supported by the Chinese state, continued to fight. In April 1975, Lon Nol went into exile. In the neighboring country of Vietnam, the Vietnam War ended with the fall of Saigon in South Vietnam. In January 1976, the Khmer Rouge fell from Phnom Penh, the capital, and promulgated the Constitution of the Democratic State of Cambodia, renaming the country Democratic Kampuchea.
