There have been a lot of players who served in the military, either before or during their career. Almost all of them, however, did so either during wartime or when we still had a draft.
Since the end of the Vietnam War, however, not many have. Mitch Harris, who pitched for the Cardinals last year, is a graduate of the Naval Academy and served. There may have been some other guys who served in the reserves, but not many. And in all cases, these guys did so before becoming established players, if indeed they ever did so.
Cardinals catcher Brayan Pena is an established player. A 12-year veteran, in fact. At age 34, it’s probably safe to say that his career is winding down. But don’t worry about him getting bored:
Pena is a Cuban native who defected when he was 16 years-old. You can read his account of that and his journey from being a poor, possibly marginal player in Cuba, to becoming a major leaguer and, eventually, a U.S. Citizen here.
It’s not common for a veteran athlete to go join the army. It’s not common for a 34 year-old to do it in any case (the age limit for the army reserves is 35). But don’t tell Pena he can’t do something. He’s already done more than anyone thought he could. He’ll probably end up a general.