During the later 1950s, he had been responsible for setting up the FLC (Front de Libération du Champa) which is an organisation seeking greater autonomy for the Cham people.

[1] In 1964, acting on the advice of a French 'handler', Kosem made overtures to the leadership of BAJARAKA, a group seeking independence for the Degar people of the Vietnamese Central Highlands.

In 1968, after internal disagreements within FULRO, Kosem was to surround the movement's headquarters with several battalions of the Royal Cambodian Army, and arrest its president Y Bham Enuol.

Kosem was also heavily involved in directing clandestine shipments of weapons from the port of Sihanoukville in Cambodia to the Viet Cong, in accordance with a secret arrangement between the Cambodian Head of State, Norodom Sihanouk, and the North Vietnamese.

In the first years of the civil war fought by Lon Nol's forces against the Khmer Rouge communists, Kosem's troops of the 5th Special Brigade were repeatedly engaged in the most active areas of insurgency, and gained a reputation for allegedly systematically slaughtering the inhabitants of pro-Khmer Rouge villages; the counterproductive results of this reputation led to the unit eventually being disbanded, according to some sources.