As documented in the
Soviet archives, Nuon Chea played a major role in negotiating the North Vietnamese invasion of Cambodia in 1970, with the intent of forcing the collapse of
Lon Nol's government:"In April–May 1970, many North Vietnamese forces entered Cambodia in response to the call for help addressed to Vietnam not by Pol Pot, but by his deputy Nuon Chea.
Nguyen Co Thach recalls: "Nuon Chea has asked for help and we have liberated five provinces of Cambodia in ten days." In 1970, in fact, Vietnamese forces occupied almost a quarter of the territory of Cambodia, and the zone of communist control grew several times, as power in the so-called liberated regions was given to the CPK [Khmer Rouge]. At that time, relations between Pol Pot and the North Vietnamese leaders were especially warm."The North Vietnamese trusted Nuon Chea more than Pol Pot or
Ieng Sary, although Chea "consistently and consciously deceived the Vietnamese principals concerning the real plans of the Khmer leadership." As a result, "Hanoi did not undertake any action to change the power pattern within the top ranks of the Communist Party to their own benefit." When the Vietnamese captured Phnom Penh in January 1979, Nuon Chea was forced to abandon his position as president of the Assembly. Prison commander
Kaing Khek Iev (also known as Comrade Duch) described Chea as "the principal man [responsible] for the killings", and stated that Chea "ordered me to kill all the remaining prisoners" at
Tuol Sleng shortly before the
regime's ouster; Chea was reportedly "furious" that Duch failed to destroy Tuol Sleng's extensive archives documenting torture and mass murder at the prison before the Vietnamese took the site. The government under Prime Minister
Hun Sen, himself a former member of the Khmer Rouge, agreed to forsake attempts to prosecute Chea, a decision that was condemned by
Western nations. American journalist
Nate Thayer, the last person to interview Pol Pot, described Nuon Chea as "probably more guilty than Pol Pot himself for the actual killings that went on while the Khmer Rouge were in power." ==Arrest and trial==