The Single Bloodiest Day in American History It's September 17, 1862 and President Abraham Lincoln needs a victory in order to issue the Emancipation Proclamation and end slavery in the South. But Robert E. Lee has other plans - invade the North. When Lee's strategy falls into the hands of the Union Army, the result is the single bloodiest day in American history at the Battle of Antietam in Sharpsburg, Maryland.