I consider it highly unlikely that David Miller ever served in the military, because of the lack of self-discipline, proper comportment, and respect for others he displays that people ordinarily learn when they serve in the military. If he screamed at his superior officers, called them names, and insulted them every time they said something to him that he didn't like--i.e., his behavior in this forum--he wouldn't have lasted two weeks. Of course, we don't know if he was thrown out of the military.
But who cares? A traitor Trumper jerk who is a veteran is not a lot different from a traitor Trumper jerk who isn't. Actually, come to think of it, a traitor Trumper jerk who is a veteran is worse, because a veteran should have respect for his country and an innate sense of patriotism--which all Trumpers lack, as they worship a traitor.
Whether or not he served his country, he is negating that service now as he pursues his agenda of divisive, false rhetoric and spreads hurtful, bogus propaganda everywhere he goes.