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JustinWarren Spahn fought at the Battle of the Bulge before he became a major league regular.
I think Yogi Berra fought too, before he made the majors.
And of course Cecil Travis had a hall of fame caliber start to this career, interrupted by the war. When he came back he was marginal at best. He said it was just him being older, but he suffered frostbite at the Battle of the Bulge and that may have contributed.
Hank Greenberg also lost a couple of years and he noted that he wasn't the same player when he came back.