[3] So Phim, after a brief period in the colonist army, joined the United Issarak Front in the fight against French protectorate of Cambodia with close ties to the Vietnamese communists and Viet Minh.
He was raised to the rank of “full right” member of the Central Committee of the Communist Party of Kampuchea in 1966, and he was appointed by Pol Pot as deputy to the political department of the People's National Liberation Armed Forces of Kampuchea and secretary of the Eastern Zone which he led until 1975 alongside Mit Phuong (after Seng Hong) and Suas Nau who was the secretary of Region 24.
Cadres imprisoned in the S-21 prison of Tuol Sleng often accused So Phim of being another traitor, inasmuch as he refused the ethnical cleansing based on tenets of Khmer racial purity.
Contrary to Seng Hong, So Phim was reluctant to resort to so much violence and questions the validity of the increasingly anti-Vietnamese policy pursued by the Angkar.
In early December 1977, Chinese Maoist Minister Chen Yonggui came to visit the East of Cambodia and insisted that the Khmer should also Learn from Dazhai in agriculture as he also inspected the "southern front" of China's ongoing battle with Vietnam.
[9] Removed from power despite his good and loyal service, So Phim founded a dissident organization in November called Authentic Revolutionary Forces of Kampuchea.
While he signed the first order which specifically targeted two Cham communities in Krouch Chhmar District, he was also considered by some to have resisted the policies of ethnic cleansing designed by the Angkar.
Realizing that they would never overthrow Pol Pot on their own power, the Khmer rouge defectors started to train new cadres with Vietnamese support at a school in Thủ Đức.