Son Sen assumed the post of supreme commander of the insurgent national army of Democratic Kampuchea on the ostensible retirement of Pol Pot in August 1985. He was removed from that position some time after the Paris peace agreements following the International Conference on Cambodia in October 1991 because of contention among the Khmer Rouge leadership over complying with its provisions; but was reported as having been reinstated to senior command in April 1994.
Son Sen was born in 1930 in southern Vietnam among the settled Cambodian minority. He was educated in Phnom Penh and then in the 1950s in Paris, where he became a member of a Marxist group of Cambodian students at whose center was Saloth Sar (Pot Pot). On his return to Cambodia, he became director of studies at the National Teaching Institute as well as a leading member of the reconstituted Communist Party of Cambodia. He fled from the capital in 1963 to escape from Prince Norodom Sihanouk's secret police and is believed to have spent time in Hanoi.
By 1971 he had become chief of staff of the Cambodian People's National Liberation Armed Forces engaged in challenging the government in Phnom Penh headed by Lon Nol. After the Khmer Rouge seized power in April 1975, he became a deputy prime minister and minister of defense until the Vietnamese invasion at the end of 1978. He continued in that role in directing the military challenge of the ousted Khmer Rouge against the Vietnamese occupation and the government established in Phnom Penh.
He was a party to the political machinery set up to implement the political settlement for Cambodia and was a Khmer Rouge member of the Supreme National Council in Phnom Penh until April 1993, when its delegation withdrew in protest at the forthcoming elections. At one time regarded as the fourth-ranking member of the Khmer Rouge hierarchy, he is believed to have engaged in factional rivalry with Pol Pot and to have been implicated in the murder of a British university teacher, Malcolm Caldwell, in Phnom Penh in December 1978.
Son Sen currently has family residing in southeast Virginia, United States after having fled Vietnam in 1983.