I'll lead off with recommending Fold3 www.fold3.com as an excellent site for military records.
As Memorial Day approaches, put some effort into finding out about your ancestors' military service. You may have a history of several consecutive generations of servicemen/women in your family. You may have ancestors who served in one or two wars. There might even be the veteran of consecutive wars.
Ask living relatives about their service or that of their parents, etc. Don't push it if they don't want to go into details.
When I was teaching, the son of a colleague came in to talk to our classes. He had recently returned from a tour in the Middle East. Naturally, one of the kids asked, "Did you kill anybody." The reply was, "That's a question you don't ask."
I'll be revisiting military research over the next week or so.