Cambodians, Montagnards, Some Viets were part of the CIDG program.
Civilian Irregular Defense Program.
They were part of a scheme to secure the remote border and highland areas. Incentives were that they could stay near home and not be drafted into the Saigon army. Also, specifically for the Montagnards, they were offered political autonomy, education, and medical care.
This is how we manned the defenses for the remote A Camps. We had Cambodia on two sides of my camp and spent time patrolling and securing the area. Mission was to interdict supply routes.
Mike Forces also used a lot of Cambodians as they were dependable, loyal and fierce mofos.
Not technically mercenaries, some refer to them as such.
By the end of 1970, SF was leaving VN and the program came under RF/PF and ARVN Rangers. They took over the old camps.
Most of the Cambodes chose to go back home. They wanted to kill Sihanouk and joined up with Pol Pot. The Khmer Rouge did not trust them and basically murdered all of them and their families.
I spent 17 months in an A Camp with about 8 to 10 other guys and a couple hundred strikers.
Quite an adventure for a kid off the farm. Highly recommended.