Actually the reason vietnam could defeat the KR in just short period of time was because there were thousands of viet agents like brother DUC who was the head of Toul Slaeng prison AKA S21, working along with the Khmer viet minh( the hun sen faction working for vietnam). Secondly, it was these groups of viet agents and khmer viet minh who fought the internal war of attrition with pol pot who tried to consolidate his grip on power. Thirdly, while pol pot people were conducting purge on these viet agents and khmer viet minh, the khmer viet minh and viet agents turned around and did their own purge against pol pot people, and each blame the others as vietnam agents. These were wide spread killing spree conducted especially in the eastern zone boarded with vietnam, which controlled by the khmer viet minh and viet agents. Then the viet had their agents like brother DUC who specializing in killing Khmer teacher, and former soldiers and intellectuals etc... In 1977, the viet activated their agents working the hun sen faction to overthrow pol pot through internal sabotage and replacing of key heads of leadership from commune to regional level. However pol pot knew of the plan and counter with his own sabotage and replacing all those viet agents and khmer viet minh cadres. That was when the viet realized that there was no other option, but set in motion to invade Cambodia to make a regime change to the one that is friendly to their hegemony and annexation of Cambodia. That is their proxy the khmer viet minh faction who they were evacuated to Hanoi after the Geneva convention in 1953.
