Paris with his doctorate in 1959, Samphan held a law faculty position at the University of Phnom Penh and started L'Observateur, a French-language leftist publication that was viewed with hostility by the government. L'Observateur was banned by the government in the following year and police publicly humiliated Samphan by beating, undressing and photographing him in public. Despite this, Samphan was invited to join Prince Norodom Sihanouk's Sangkum, a 'national movement' that operated as the single political party within Cambodia. Samphan stood as a Sangkum deputy in the 1962, 1964 and 1966 elections, in which the lattermost the rightist elements of the party, led by Lon Nol, gained an overwhelming victory. During his tenure, according to Minister of Information Trinh Hoanh, he was known as "a modest man of relatively humble origins who, even while minister of economy, refused to be driven in a Government limousine". In 1966, he then became a member of a 'Counter-Government' created by Sihanouk to keep the rightists under control. However, Samphan's radicalism led to a split in the party and he had to flee to a jungle after an arrest warrant was issued against him. At the time, he was even rumoured to have been murdered by Sihanouk's security forces. In the Cambodian coup of 1970, the National Assembly voted to remove Prince Sihanouk as head of state, and the Khmer Republic was proclaimed later that year. The Khmer Rouge, including Samphan, joined forces with the now-deposed Prince Sihanouk in establishing an anti-Khmer Republic coalition known as the National United Front of Kampuchea (FUNK), and an associated government: the Royal Government of the National Union of Kampuchea (GRUNK). In this alliance with his former enemies, Samphan served as deputy prime minister, minister of defence, and commander-in-chief of the Cambodian People's National Liberation Armed Forces, the GRUNK military. FUNK defeated the Khmer Republic in April 1975 and took control of all of Kampuchea. During the years of Communist Democratic Kampuchea (1975–1979), Samphan remained near the top of the movement, assuming the post of president of the central presidium in 1976. His faithfulness to Pol Pot meant that he survived the purges in the later years of the Khmer Rouge rule. His roles within the party suggest he was well entrenched in the upper echelons of the Communist Party of Kampuchea, and a leading figure in the ruling elite. In 1985, he officially succeeded Pol Pot as