The Battle of Antietam, also called the Battle of Sharpsburg, was a battle between Union Gen. George B. McClellan's Army of the Potomac and Confederate Gen. Robert E.Lee's Army of Northern Virginia. It took place on September 17, 1862, near Sharpsburg, Maryland, and was the bloodiest day of the Civil war.

With about 22,717 casualties, dead, injured, or missing, the Battle lasted about 12 hours, and it was a fight between the Union Army and the Confederates. The Union Army won, but it suffered more casualties than the Confederates, including six generals killed. Three were from the Union Army, and the others were from the Confederates' side. A dozen more generals were wounded.

The Battle of Antietam is cited as having protected the Federal Capital from invasion, influencing Great Britain not to recognize the Confederacy and shaping the future of warfare in the Civil War that ended on April 9, 1865.

Following the Union Army's victory at Antietam, President Lincoln used the occasion to issue the Preliminary Emancipation Proclamation on September 22, 1862. The Proclamation announced that Lincoln would free all enslaved people outside Union-controlled territory if the Confederates failed to lay down their arms.