The original series of Star Trek had no ground forces. The vision was that the ships of the fleet would explore and enlighten. Of course, the universe is a nasty place and many Security Officers went out and proved it. The first real use of actual military forces – ship borne infantry – was the MACO team assigned to the NX-01 as a support group and they proved their worth. Military Assault Command, Operations evened the odds and settled the score on numerous missions under Archer’s command. They became a valuable part of the fabric of Star Trek and they remain so in STARFLEET today.
Most of the MACO contingent aboard hold their position as reservists – ready to act when duty calls – though you can be a full-time MACO if you so desire. As full crew members in both services, they answer that call for charity, comradery, and service, because that’s really what STARFLEET is all about.