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
Sterling Price, with 18,000 men, lays siege to Lexington, Missouri, with a federal force of 3.600 under Colonel James Mulligan. After fighting intensified on September 19, Mulligan surrendered on the 20th.