The long-term impact of the Battle of Antietam on U.S. history was that it led to the abolition of slavery.

The battle gave President Lincoln the political and military leverage necessary to issue the Emancipation Proclamation soon after. This pivotal document proclaimed freedom for enslaved individuals in Confederate states, transforming the war into a struggle for liberty and human rights. It not only changed the moral and political aims of the conflict but also set the stage for the eventual nationwide abolition of slavery, profoundly impacting the trajectory of American history.