The support branches of the services are what allow the 'sharp end' to continue fighting. They provide replacement equipment, food and personnel. They provide the required maintenance for all the machinery, health care and pay for the troops, information for the commanders, communications, discipline within the ranks and in the currently occupied territory, and legal advice.
I'm going to spend the next few months discussing the (usually unsung) support services, mostly from a U.S. Army point of view, since that is what I am most familiar with. (Unless I get shrieks of agony, begging me to stop.)
In honor of the upcoming reunion of my dad's WWII unit, the 821st Military Police Company, I'll start off with the 'cops.'
The very concept of being in the military denotes very strict discipline. Everyone has a job to do and a place to be. Everyone is carefully monitored by the leaders directly above him; and doesn't dare step out of line. Units will go where they are told to go.
The foregoing, unfortunately, is only partly true. Normal confusion will ensure that some people, or units, will not be in the right place or doing the right thing. Normal human nature will ensure that some people commit crimes or lose interest in membership and attempt to leave. The larger an army is, the more of a problem this becomes, and the more need there is for a force committed to maintaining order.
Naval forces have usually had a somewhat simpler task, because their personnel are limited to the confines of their ships for a large portion of their service. The Master at Arms on board a ship doesn't have to chase anyone very far. However, as ships have become larger; with battleships and carriers shipping crews of several thousand; the job has become more difficult. On some ships, it's much like serving on a metropolitan police force.
Law enforcement units have been around since armies were first formed. More often than not, they have been regular combat units simply assigned to the duty for a limited time. As armies became more 'civilized' (read, more bound by written regulations) formations specifically dedicated to enforcement of the regulations were formed; but these formations were seldom permanent.
