The Battle of Antietam, September 1862 near Sharpsburg, Maryland was the bloodiest single-day battle of the War, with 23,000 casualties on both sides.
Emboldened by the success of Second Manassas, Lee decided to take the war into enemy territory. Jefferson Davis and others believed the prospect of foreign recognition would increase if they could have a victory in enemy territory. Unfortunately it was not to be.
Lee and the Army of Virginia  had two Corps, one under Longstreet and the other under Jackson,  Cavalry under JEBC Stuart, and Artillery under Pendleton.. McClellan, and the Union Army of the Potomac  had the following Corps: Hooker, Sumner, Fitz John Porter, Franklin, Burnside and Mansfield, and  Cavalry under Pleasonton. 
The carnage was incredible. The fighting in the Cornfield was unimaginable and the Bloody Lane was filled with bodies by the time it was over. The Battle was a tactical draw, but it was considered a turning point of the War and a victory for Lincoln, as it ended Lee’s first invasion of the North.