The Battle of Antietam

Antietam was a completely different story - Lee had a full-sized army that was spent, exhausted, and hungry.  They were winning and had pushed McClellan from Richmond, the Confederate capital.  He was confident invaded Maryland, also hoping he could convince to join the Confederate cause.  He gave a speech from Jefferson Davis telling Maryland they were liberated from the Union.

He was successful in his invasion until two Union soldiers found 3 cigars wrapped in some paper.  The paper was a copy of Lee's battle plan.  McClellan takes 18 hours to plan his move, during which time a Maryland citizen who was a Confederate supporter rode hard to Lee and informed him.  Lee is able to put soldiers in place to slow McClellan's approach and then set up a good defensive battleground - at Antietam.

Watch how this battle unfolds, with the two largest armies in the divided nation:

https://www.battlefields.org/learn/maps/antietam-animated-map

This battle was the bloodiest single day in the entire civil war, and in all of US history.  Many blame McClellan for not pushing through and following Lee on his retreat, believing that they could have fully ended the conflict here.

Still, the Confederates did retreat, and Lincoln called it a victory for the Union - and announced the Emancipation Proclamation.