Quoc: McClellan had halted Lee's invasion of Maryland, but Lee was able to withdraw his army back to Virginia without interference from the cautious McClellan. McClellan's refusal to pursue Lee's army led to his removal from command by President Abraham Lincoln in November. Although the battle was tactically inconclusive, the Confederate troops had withdrawn first from the battlefield, making it a Union strategic victory. It was a sufficiently significant victory to give Lincoln the confidence to announce his Emancipation Proclamation, which discouraged the British and French governments from pursuing any potential plans to recognize the Confederacy.

Who won the civil war battle of antietam?
HW Chung: OK, some thoughts:
The relevant information to answer the above question is: Although the battle was tactically inconclusive, the Confederate troops had withdrawn first from the battlefield, making it a Union strategic victory. The answer is Union.