The 18 Deadliest Battles in American Military History

[Juiceman]

Antietam was the bloodiest single day, with the bloodiest battles of the ACW in order of total causalities:  Gettysburg, Chickamauga, Spotsylvania Court House, Chancellorsville, The Wilderness, most of the battles were 2-3 days with Spotsylvania being a 2 week battle.  
One of the great things about living just outside of Philly in the late 80's was being a short drive to many Civil War Battlefields, if you ever get a chance go visit Antietam, walking around that battlefield (Bloody Lane, Burnsides Bridge, the Cornfield) you can see why it was the bloodiest day in US History.