What was crucial about the battle of Antietam?

Answer 1

It was the Northern victory that Lincoln was waiting for, so that he could issue the Emancipation Proclamation without sounding as though it was a desperate measure.

This made the North feel that they were fighting a moral crusade, and made it impossible for other countries to help the Confederates without looking pro-slavery.

Answer 2

It was the unexpected Northern victory that gave Lincoln the credibility to issue the Emancipation Proclamation.

This turned the war into an official crusade against slavery, and made it ethically impossible for Britain and France to aid the Confederates.

Answer 3

This slim margin of the Northern victory interrupted Lee's impressive string of victories, which had brought the British close to granting official recognition to the Confederacy. They may had been treating Lee's raid into Maryland as the test of Confederate viability. Now the momentum was suddenly lost, and British intervention was put on the back-burner.

In military terms the battle was a technical draw. With the Army of Northern Virginia so far from home, it would have had to retreat back to Virginia even if the Confederates had won.

President Lincoln now had the credibility to issue the preliminary Emancipation Proclamation without making it look like a desperate gesture. Several members of his cabinet had suggested he wait for a Northern victory.

Answer 4

The best answer to this question is to note that the Battle of Antietam was not a significant one in terms of the outcome of the US Civil War. There were many battles yet to be fought. Confederate General Lee had hoped for perhaps an intervention by the British. A "peace treaty" developed by Great Britain may have ended the fighting. There was no such intervention. This did not keep both France and Britain from selling warships and firearms to the South.