Mercs would make probably the most sense.
I have Dutch as Army SF, maybe something like Project Delta or part of something like MACV-SOG which would get him contacts out of all the branches and the CIA. Having success there as a commander, then getting out and doing odd jobs for the US government with his hand picked crew afterwards.
But they could still be operatives in something like Delta or an Army SF CIF team.
There are lots of Army references, and his rank and his familiarity of fieldcraft would make him most likely former Army or Marine. Navy O-4 is not Major. And even though the AF has the ranks, and even a whole SOF career field dedicated to rescuing people, I don't know that their training includes setting up deadfall traps and running claymores through trees.