Final answer:
The Battle of Antietam was critical because it was the bloodiest single-day battle in American history, shifted the momentum of the Civil War, and led to the Emancipation Proclamation.
Step-by-step explanation:
The Battle of Antietam is considered critical for several reasons. Firstly, it was the bloodiest single-day battle in American history, with approximately 23,000 casualties. Secondly, it shifted the momentum of the Civil War in favor of the Union, as General Robert E. Lee's Confederate army was halted and forced to retreat. Lastly, it led to President Abraham Lincoln issuing the Emancipation Proclamation, which declared the freedom of all slaves in Confederate territory.
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