The correct answer is the Battle of Antietam.
The Battle of Antietam was the bloodiest battle in the Civil War. Roughly 22,000 were killed. However, this battle gave the Union a victory in terms of strategic location. This "victory" gave Lincoln the confidence to issue the Emancipation Proclamation. The Emancipation Proclamation stated that slaves "in all rebellious states" (meaning the Confederacy). This did not sit well with the Confederates and practically all the states ignored it since they considered themselves part of another country anyways.