In December 1977, the Vietnamese launched a punitive attack into eastern Cambodia, quickly routing the eastern zone troops including Heng Samrin's Division 4 and further convincing Pol Pot of So Phim's treachery. Son Sen was sent to the eastern zone with center zone troops to aid the defense. In January 1978, following the Vietnamese withdrawal, a purge of the eastern zone began. In March, So Phim called a secret meeting of his closest subordinates advising them that those who had been purged were not traitors and warning them to be wary. During the next month more than 400 eastern zone cadres were sent to Tuol Sleng while two eastern zone division commanders were replaced. During May eastern zone military leaders were called to meetings where they were arrested or killed. So Phim was called to a meeting by Son Sen but refused to attend, instead sending four messengers who failed to return. On 25 May, Son Sen sent two brigades of troops to attack the eastern zone and capture So Phim. Unable to believe he was being purged, So Phim went into hiding and attempted to contact Pol Pot by radio. A meeting was arranged, but instead of Pol Pot a group of center soldiers arrived, and So Phim committed suicide; the soldiers then killed his family.