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 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."