What was the significance of Antietam?

It was a Union victory, largely accidental, that had not been expected by anyone, including the British who were treating this campaign as the test of Confederate viability, in advance of granting recognition and sending aid.

This victory gave Lincoln the credibility to issue his Emancipation Proclamation, turning the war into an official crusade against slavery. This meant that free nations abroad could no longer help the South without looking pro-slavery themselves.

Overseas support was never likely after this, and the Confederates realised they would have to go it alone.