Antietam, Battle ofor Battle of Sharpsburg , September 17, 1862, a major battle of the American Civil War. It was the bloodiest single day's battle of the war. It was fought in western Maryland at the village of Sharpsburg, between Antietam Creek and the Potomac River. The site, called Antietam National Battlefield, is a unit of the National Park System.

The Army of Northern Virginia, under General Robert E. Lee, invaded Maryland on September 5. On September 17 approximately 46,000 Union troops of the Army of the Potomac, commanded by Major General George B. McClellan, met 40,000 Confederates at Sharpsburg. The Union forces lost some 13,000 men in the battle, the Confederates about 10,000. McClellan had fresh troops in reserve, but failed the next day to renew the engagement. Soon after, he was relieved of his command.

Although the battle was a draw tactically, it amounted to a Union victory because the Confederates fell back into Virginia. President Lincoln, who had been waiting since July for a Union victory to provide a suitable occasion, issued a preliminary version of the Emancipation Proclamation five days after the battle. (This preliminary proclamation was a warning to the Southern states that he intended to issue the final proclamation on January 1, 1863. )