Hun Sen was educated at a Buddhist monastery in Phnom Penh. In the late 1960s he joined the Communist Party of Kampuchea and in 1970 joined the Khmer Rouge. During the regime of Pol Pot (1975–79), when an estimated two million Cambodians lost their lives, Hun Sen fled to Vietnam, joining troops there opposed to the Khmer Rouge. He returned to Cambodia after the Vietnamese installed a new government in 1979 and was made minister of foreign affairs. He became prime minister in 1985.