In Batman: Year One, the animated adaptation, there is a point where Jim Gordon is beating up Flass, and as that about to go down, he gives him a baseball bat.
The voice-over says something along the lines of "It's been a while since I had to take down a former Green Beret, still he deserves a handicap", after which Gordon gives Flass the bat, and proceeds to kick the living crap out of him.
What did Jim Gordon do before coming to Gotham, that he had to take down Green Berets, that it comes to him so easily?