sep 17, 1862 - Battle of Antietam

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The Battle of Antietam gave the Union army confidence that it could beat the Confederate army, and led to Lincoln issuing the Emancipation Proclamation. The battle is also resulted in the deaths of around 20,000 soldiers, making it the deadliest one-day battle in American history.
Sources:
https://www.battlefields.org/learn/civil-war/battles/antietam

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