Loved Sid Meier's Gettysburg, one of the first non-FPSes I played online a bunch. It was also quick, no resource management or anything really, just historically accurate troop reinforcements and placements. Though I always played the full battle mode instead of the scenarios. And at the time it seemed like an insane number of on-screen troops.

It was easy as fuck as the Confederates though, you just had to not fuck up the first two encounters, which in real life they did. Especially if you ran a long flank with Early's troops when they arrived instead of using the road like the game (and history) told you to. (Since you had already prevented the Union advance into the town and the game always tried to flank you by heading north because of better cover. Which then put them right inbetween your two forces when Early arrived.)
Never actually played Antietam. Wouldn't mind those on GOG in a pack like Alpha Centauri. Steam is fine too.