The battle along Antietam Creek, about 60 miles north of Washington, on Sept. 17, 1862, was the bloodiest day of combat on U.S. soil. More than 23,000 Union and Confederate soldiers were reported killed, wounded or missing during 12 hours of fighting with no clear winner.
Gen. Robert E. Lee withdrew his troops to Virginia the next day. The Confederate retreat gave President Abraham Lincoln the political strength to issue a preliminary Emancipation Proclamation five days after the battle.
Images of the battle, known in the South as the Battle of Sharpsburg, were captured by photographer Alexander Gardner, an associate of photographer Matthew Brady. It has been argued that some of the scenes in Gardner's photos of the Civil War had been posed.
This photograph shows President Lincoln and Gen. George B. McClellan in the general's tent.