The Battle of Antietam marked the culmination of Confederate General Robert E. Lee's first invasion of the Northern states. Guiding his Army of Northern Virginia across the Potomac River in early September 1862, the great general daringly divided his men, sending half of them, under the command of General Thomas "Stonewall" Jackson, to capture the Union garrison at Harper's Ferry.
President Abraham Lincoln put Major General George B. McClellan in charge of the Union troops responsible for defending Washington, D.C., against Lee's invasion. McClellan's Army of the Potomac clashed first with Lee's men on September 14, with the Confederates forced to retreat after being blocked at the passes of South Mountain. Though Lee considered turning back toward Virginia, news of Jackson's capture of Harper's Ferry reached him on September 15. That victory convinced him to stay and make a stand near Sharpsburg, Maryland.
Over the course of September 15 and 16, the Confederate and Union armies gathered on opposite sides of Antietam Creek. On the Confederate side, Jackson commanded the left flank with General James Longstreet at the head of the center and right. McClellan's strategy was to attack the enemy left, then the right, and finally, when either of those movements met with success, to move forward in the center.
When fighting began in the foggy dawn hours of September 17, this strategy broke down into a series of uncoordinated advances by Union soldiers under the command of Generals Joseph Hooker, Joseph Mansfield and Edwin Sumner. As savage and bloody combat continued for eight hours across the region, the Confederates were pushed back but not beaten, despite sustaining some 15,000 casualties. At the same time, Union General Ambrose Burnside opened an attack on the Confederate right, capturing the bridge that now bears his name around 1 p.m. Burnside's break to reorganize his men allowed Confederate reinforcements to arrive, turning back the Union advance there as well.
By the time the sun went down, both armies still held their ground, despite staggering combined casualties--nearly 23,000 of the 100,000 soldiers engaged, including almost 4,000 dead. McClellan's center never moved forward, leaving a large number of Union troops that did not participate in the battle. On the morning of September 18, both sides gathered their wounded and buried their dead. That night, Lee turned his forces back to Virginia. His retreat gave President Lincoln the moment he had been waiting for to issue the Emancipation Proclamation, a historic document that turned the Union effort in the Civil War into a fight for the abolition of slavery.