At the end of 1976, following disappointing rice harvests in the northwestern zone, the party center ordered a purge of the zone. Troops from the western and southwestern zones were ordered into the northwestern zone. Over the next year, troops killed at least 40 senior cadre and numerous lower ranking leaders. The chaos caused by this purge allowed many peasants to escape the zone and seek refuge in Thailand. In 1977, the center began purging the returnees, sending 148 to Tuol Sleng and continuing a purge of the ministry of foreign affairs where many returnees and intellectuals were suspected of spying for foreign powers. In January, the center ordered eastern and southeastern zone troops to conduct cross-border raids into Vietnam. In March 1977, the center ordered So Phim, the eastern zone commander, to send his troops to the border; however with class warfare purges underway in the eastern zone, many units staged a mutiny and fled into Vietnam. Among the troops defecting in this period was Hun Sen. On 10 April 1977 Hu Nim and his wife were arrested. After three months of interrogation at Tuol Sleng, he confessed to working with the CIA to undermine the revolution following which he and his wife were executed. In July 1977, Pol Pot and Duch sent So Phim a list of "traitors" in the eastern zone, many of whom were So Phim's trusted subordinates. So Phim disputed the list and refused to execute those listed, for the center this implicated So Phim as a traitor. In October 1977, in order to secure the Thai border while focusing on confrontation with Vietnam, Nhim Ros, the northwestern zone leader, was blamed for clashes on the Thai border, acting on behalf of both the Vietnamese and the CIA.