one zone frequently were sent to another. These efforts were considered to be essential by the central governing group as to control regional secretaries and their dissident or ideologically impure cadres. A practice that eventually culminated in widespread bloody purges decimating the ranks, undermined the morale and considerably contributed to the rapid collapse of the regime. As author Elizabeth Becker noted, "in the end paranoia, not enemies, was responsible for bringing down the regime." Cambodian–Vietnamese border tensions Tensions between Cambodia and Vietnam have been going on for hundreds of years. They first peaked during the 19th century when only the establishment of the French Protectorate of Cambodia prevented the inevitable incorporation into a Vietnamese empire. French colonial administrators established numerous administrative zones and borders such as the Brévié Line, with little regard to historic and ethnic considerations. Virtually all of these re-drawings resulted in territorial gains for Vietnam, who since independence after 1945 refuse to re-negotiate with Cambodia referring to the authority of the colonial calibrations. First clashes between the RAK and Vietnam's NVA date back to 1970, as Khmer Rouge units fired on North Vietnamese troops. Reports of engagements of growing intensity continued, particularly after 1973. The North Vietnamese initially chose to ignore the incidents because they regarded the sanctuaries in Cambodia as vital for their domestic war. After the communist victories of April and May 1975, Khmer Rouge border raids increased, that included massacres of villagers. Democratic Kampuchea attempts to capture several disputed insular territories in the Gulf of Thailand (e.g. Thổ Chu and Phú Quốc) ended, apart from high civilian losses, in failure. Deteriorating Cambodia–Vietnam relations reached a low on 31 December 1977 as Radio Phnom Penh reports a "ferocious and barbarous aggression launched by the 90,000 [troops of the] Vietnamese aggressor forces against Democratic Kampuchea", denounced the "so-called Socialist Republic of Vietnam" and announced a "temporary severance" of diplomatic relations. Escalating rhetoric border skirmishes erupted into pitched battles in the summer and the fall of 1978. Major engagements were reported from Svay Rieng Province, Kampong Cham Province and Ratanakiri Province. In November 1978 Vietnamese forces launched a sustained operation on Cambodian soil in the area of the town of Snuol and Memot in Kratié Province. This action cleared a liberated zone where anti-Khmer Rouge Cambodians could launch a broad-based political movement that opposed the inhumane Pol Pot regime. On 2 December 1978 the Kampuchean National United