The event that marked the end of the Battle of Antietam was Lee’s retreat.

After a grueling day of intense fighting on September 17, 1862, the Confederate Army, facing significant casualties and unable to achieve its objectives, began withdrawing across the Potomac River into Virginia. Despite the Union Army’s considerable losses, Lee's decision to retreat allowed him to preserve his forces, making the battle a strategic victory for the Union. This retreat ultimately ended the fighting at Antietam, leading to significant political consequences, including the issuance of the Emancipation Proclamation.