Beginning early on the morning of September 17, 1862, Confederate and Union troops in the Civil War clash near Maryland’s Antietam Creek in the bloodiest single day in American military history.

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. Over the course of September 15 and 16, the Confederate and Union armies gathered on opposite sides of Antietam Creek.

Fighting began in the foggy dawn hours of September 17. 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.

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 more than 3,600 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.