Using Real Life Military Units in Games

Hello,
So as the title says I was thinking about game ideas the other day and was wondering if you are allowed to use specific military units in game. For example if I wanted to create a game all about the Air Force Pararescuemen. Would I be allowed to do that? Also how specific could I get? Down to exact unit names and numbers or does it have to be broader? Anything else I should know about it if I did do this?

Thanks for looking,
NickHaldon