Which American Civil War battle is considered the "bloodiest day" in United States history?

The Battle of Antietam was fought on September 17, 1862 near Sharpsburg, Maryland and Antietam Creek. The Army of the Potomac under the command of General George B. McClellan attacked Robert E. Lee's Army of Northern Virginia. Lee had invaded Maryland from Virginia during the Maryland Campaign in an attempt to penetrate as far north as Pennsylvania to cut the supply line to Washington, D.C. He also intended to bring the war to the north, which had been largely unscathed in the conflict. Lee also hoped to disrupt Northern morale to influence the 1862 congressional elections. A Confederate victory on Northern soil would increase the likelihood of British and French recognition of the rebellious states.

Despite being outnumbered by more than 2-1, Lee managed to successfully repel three successive Union attacks, using interior lines, and deploying nearly all of his force against the cautious McClellan who refused to utilize his ample reserves in "force concentration" because of his mistaken belief that he was outnumbered. The total casualties of both sides, (killed, wounded, and missing), is given at 22,717, making it the bloodiest day in U.S. history.

Lee retreated back into Virginia. Though the battle was tactically inconclusive, the retreat enabled President Lincoln to issue the Emancipation Proclamation which further denied the Confederacy the chance of foreign recognition.