Final answer:
The Battle of Antietam led to the issuance of the Emancipation Proclamation, aligning the Union's cause with British anti-slavery values and discouraging England and France from recognizing the Confederacy, isolating the South and impacting the course of the Civil War.
Step-by-step explanation:
The Battle of Antietam played a crucial role in the Civil War, particularly with regard to international relations. It was the bloodiest single day in American history and although it was not a decisive Union victory, it provided President Abraham Lincoln with the opportunity to issue the Emancipation Proclamation.
Great Britain's earlier abolition of slavery strongly influenced their stance on the American Civil War. The Emancipation Proclamation further aligned the Union with British anti-slavery sentiments, and as a result, Britain moved further away from recognizing the Confederate States.