when Saloth Sar took over the Communist Party leadership, a number of leftists were beginning to escape Phnom Penh for the countryside; Sen went underground and was to follow them in 1964, hidden in the car boot of a Chinese diplomatic vehicle. Sen was initially to join Saloth Sar and other former colleagues at a Vietnamese Communist military base called Office 100, located in the border areas of Cambodia. There are indications that he was soon sent to the remote, malarial Ratanakiri Province, in the far north-east of the country, in order to build up anti-government activity amongst the Khmer Loeu tribesmen. By the later 1960s, Sen – with increasing experience of guerrilla warfare – had built up a reputation as a talented field commander, and had risen to a senior rank within the Party. In 1968, he was responsible for several successful uprisings in the south-west of the country, and was reported to be the chief of the political committee for Kampot, Takéo and Kampong Speu. Chief of Staff of Khmer Rouge forces After the Cambodian coup of 1970, and the subsequent establishment of the Khmer Republic under Lon Nol, Sihanouk was to join with his former Communist enemies in forming the GRUNK, a Beijing-based government-in-exile. The Khmer Rouge's forces were soon to be swelled by thousands of new recruits, attracted in part by Sihanouk's name. Son Sen, whilst not a prominent member of the GRUNK, had considerable power thanks to his high position in the government's forces on the ground – CPNLAF, the Cambodian People's National Liberation Armed Forces. Along with Ieng Sary, he was responsible for the North-Eastern Zone, the stronghold of the Party 'Centre'. By 1972 Son Sen had become Chief of Staff of the Khmer Rouge forces. Despite occasional criticisms – his colleagues found his style peremptory and his point of view "bourgeois", and he was openly criticised by Hou Yuon at the Party's 1971 Congress – his devotion to Saloth Sar and his closeness to the Party 'Centre' guaranteed his senior position. Democratic Kampuchea After the Khmer Rouge captured Phnom Penh in April 1975, Son Sen was appointed Deputy Prime Minister with responsibility for Defense. Responsible for internal security, he also oversaw the Santebal – the Khmer Rouge secret police. As such he monitored the operations of the infamous S-21 prison at Tuol Sleng, and engaged actively in the design of its interrogation and