To refer to these men as mercenaries or purely as civilians is inaccurate.
I was picked up by DoS/PM/WRA (Weapons Removal and Abatement) years back to work under RONCO with Humanitarian Demining. They were adamant about required skills and especially military background. Language abilities would be a plus.

I know several folks out of DS, most are or were cops, special agents and almost always former military officers and NCOs.

Welcome Aboard !

The linkage is quite simple: if the US government decides it is necessary to armed forces on the ground, then the US government needs to equip train and deploy US government forces to do that job. Contractors are offered as a "cheap" or "short term" fix for what are often self-created shortfalls.
On the other hand, Tom hits a sore point (he does that often). Once in place and other than a monthly paycheck, training, follow-on training and needed (relatively significant and yes, expensive) equipment never showed. Keep it cheap one said to me from the onset.

Regards, Stan