It is a easier way to automatize regiment replenish. 

A military region is a region of several cities where regiments normally stationed, replenished and get back to when war ends.  It should be presented on "all military force" panel, grouping up regiments that came from the same military regions.

This is how it works: there will be a “plus” sign on regiment units slot for player to choose what unit they wanted to replenish, then the cities with the most population in the military region start producing that units, then AI walk them to where the regiment stationed. When regiment are trying to get replenishment, they could only station where they are or retreat to friendly territory.