The Union Army of the Potomac repulsed a Confederate offensive at the Battle of Antietam in Maryland, for a Union strategic victory. Lee was forced to retreat, allowing the North to declare the battle a Union victory, though Union forces failed to follow up on their surprise success and decisively defeat Lee’s army. Antietam had the highest number of casualties in a single day (a combined 23,000) and was the sixth costliest battle of the Civil War and the first major battle on Northern soil.