Battle of Sharpsburg (Confederate)
Battle of Antietam (Union)
Army of the Potomac under McClellan [US] defeats the Army of Northern Virginia under Lee [CS], resulting in the bloodiest day in American history.
Union losses:12,401 men
Confederate losses:10, 406
Skirmishes at Shepherdstown, Ashby's Gap, Williamsport, and Hagerstown, as Confederates under A. P. Hill covered the retreat of the Army of Northern Virginia from Sharpsburg. Lee would keep a heavy cavalry presence in the area until October.