Following the Cambodian coup of 1970, in which Prince Norodom Sihanouk was deposed by General Lon Nol, Sutsakhan continued his career within the army, now renamed the Khmer National Armed Forces (FANK), and oversaw its substantial expansion in 1971–72.

He served as Minister of Defence several times, was the commander of the FANK Special Forces, and had a good reputation amongst U.S. diplomats and military advisors as a competent senior officer, as well as a capable and non-corrupt politician.

[4] After the U.S. embassy staff and acting President Saukam Khoy departed Phnom Penh on April 12 during Operation Eagle Pull, a seven-member Supreme Committee, headed by Lieutenant-General Sak Sutsakhan, assumed the authority over the collapsing Republic.

The KPNLF was initially a coalition of various disparate 'resistance' and even semi-bandit groups based in the refugee camps along the Thai-Cambodian border; however the recruitment of Sutsakhan, who arrived from the US in 1981, helped to lend considerable legitimacy to the cause.

The resulting split in the KPNLF hampered the operations of its armed forces: despite some initial successes in the northwest of Cambodia, the KPNLAF was shattered by a Vietnamese offensive in 1984–1985 and was largely restricted to guerrilla warfare after this point.