torture procedures. Son Sen's role meant that he was particularly closely implicated in the many thousands of deaths that occurred in this period due to the arrest and execution of perceived enemies of the regime. His memoranda to his subordinate Kang Kek Iew, and notes taken during study sessions overseen by Sen for S-21 cadres, reveal his continued interest in history, deeply anti-Vietnamese views, and revolutionary zeal, along with a "schoolmasterish attention to detail". These were combined with a brutally ruthless streak; Sen was reputed to have once been so angry with Foreign Minister Ieng Sary that he unsuccessfully attempted to have him murdered. The Party's paranoia with regard to security, and its obsessive secrecy, led many senior Khmer Rouge cadres to be identified in internal documents by pseudonyms or numbers: Son Sen was often referred to as "Khieu", and his wife as "At". He was also identified as "Brother 89" on letters and memoranda. Sen's other main duties in this period involved reorganising the CPNLAF forces into a cohesive national army, the Revolutionary Army of Kampuchea. By 1977–78, he was overseeing an increasing series of clashes with Vietnamese forces along the border, as well as mounting a massive purge of Eastern Zone cadres considered to have Vietnamese links. As the ongoing war with the Vietnamese began to go increasingly badly for the Democratic Kampuchean forces, however, Sen began to fall under suspicion himself, and may well have become a victim of his own security apparatus if the 1979 Vietnamese invasion had not intervened. In particular, he had been implicated by the Party's internal security in several "traitorous" activities including the 1978 murder of the British academic Malcolm Caldwell. Commander of the NADK Following the Vietnamese invasion and the establishment of a pro-Vietnamese regime in Phnom Penh, Sen was to re-establish his control over the Khmer Rouge forces, now operating against the Vietnamese and the forces of the People's Republic of Kampuchea from bases in the Cardamom Mountains. By August 1985, when the 'retirement' of Pol Pot was officially announced, Son Sen assumed supreme command of the National Army of Democratic Kampuchea, as it was now called. During this period Sen had occasional contact with the commanders of the other armed groups within the Coalition Government of Democratic Kampuchea; the Khmer People's National Liberation Armed Forces under Sak Sutsakhan, and the ANS armed wing of the royalist FUNCINPEC, commanded by