The Battle of Antietam on September 17, 1862 is the bloodiest day in American military history. Now, exactly 150 years later, a panel of historians discusses the events of that day.

Leading his Confederate troops into Maryland for their first fight on Union soil, Robert E. Lee was met at Antietam Creek by George McClellan’s federals. The battle claimed 23,000 casualties and resulted in a standoff. But after that the Union believed it could win, giving President Abraham Lincoln the confidence to issue his Emancipation Proclamation.