[3] In 1970 he was promoted to lieutenant colonel in the Khmer National Armed Forces (FANK) and assumed the command of the 2nd Brigade at Prey Sar near Phnom Penh.

His unit was involved in the disastrous Operation Chenla II, an attempt to clear National Route 6 in Kampong Thom Province in 1971.

Journalist Elizabeth Becker later described a visit to his headquarters on Highway 4 in January 1973 with cameraman Neil Davis, seeing the heads of Khmer Rouge soldiers on stakes along the road.

[4]: 136  Throughout the early months of 1975 he supervised the defense of Phnom Penh, and Khmer Rouge Radio reported that on 20 February he was "severely injured" in combat at Dei Eth.

[8] In May 1977 he flew to Paris and helped to organize a political group in cooperation with non-communist resistance forces under former Prime Minister Son Sann.

[9]: 8  On 1 February 1979, Dien Del flew to Thailand to form the Khmer People's National Liberation Armed Forces (KPNLAF).

Minister of Defense under Sihanouk, he had been Head of State of the Khmer Republic during its final days and had a reputation for decisiveness and incorruptibility that lent legitimacy to the KPNLF.

[11] This setback, which was blamed on Son Sann for his alleged meddling in military matters (particularly his unwillingness to cooperate with Sihanouk's forces), aggravated long-standing conflicts within the KPNLF.

He said, however, that General Sak would remain as Commander-in-Chief of the Joint Military Command, which was launched in January 1986 reportedly as a concession to the dissident group.

[13] After shedding his monk's robes in late 1986, General Dien continued as deputy commander-in-chief, directing combat operations against the Vietnamese until their withdrawal from Cambodia in 1990.