The Khmer Rouge captured Phnom Penh, the capital of the Khmer Republic, ending the Cambodian Civil War and establishing the socialist state of Democratic Kampuchea.

Khmer Rouge is the name that was popularly given to members of the Communist Party of Kampuchea (CPK), and by extension to Democratic Kampuchea, which ruled the country between 1975 and 1979. The name was coined in the 1960s by Norodom Sihanouk to describe his country's heterogeneous, communist-led dissidents, with whom he allied after the 1970 Cambodian coup d'état.

The Fall of Phnom Penh was the capture of Phnom Penh, capital of the Khmer Republic, by the Khmer Rouge on 17 April 1975, effectively ending the Cambodian Civil War. At the beginning of April 1975, Phnom Penh, one of the last remaining strongholds of the Khmer Republic, was surrounded by the Khmer Rouge and totally dependent on aerial resupply through Pochentong Airport.

The Khmer Republic was a Cambodian state under the United States-backed military dictatorship of Lon Nol from 1970 to 1975. Its establishment was formally declared on 9 October 1970, following the 18 March 1970 coup d'état which saw the overthrow of Norodom Sihanouk's government and the abolition of the Cambodian monarchy.

The Cambodian Civil War was a civil war in Cambodia fought between the Communist Party of Kampuchea against the government of the Kingdom of Cambodia and, after October 1970, the Khmer Republic, which had succeeded the kingdom after a coup. The conflict was part of the Second Indochina War (1955–1975).

Democratic Kampuchea was the official name of the Cambodian state from 1976 to 1979, under the government of Pol Pot and the Communist Party of Kampuchea (CPK), commonly known as the Khmer Rouge. The Khmer Rouge's capture of the capital Phnom Penh in 1975 effectively ended the United States-backed Khmer Republic of Lon Nol.