the battle of antietam, also called the battle of sharpsburg, was not only the bloodiest day of the american civil war, it was the single bloodiest day in american history. most of the fighting occurred on september 17, 1862 between antietam creek and the town of sharpsburg, maryland. the events leading to the battle were somewhat strange. earlier, a copy of order no. 191 had been used to bundle a confederate officer's cigars. it was dropped and found by union soldiers. they were taken to mcclellan who now knew exactly what lee's battle plan was. however, a southern sympathizer told lee that mcclellan had a copy of the order. so mcclellan's advantage was lost. because mcclellan took his time after the battle of south mountain, lee regrouped around sharpsburg. the battles started on the morning of the 16th. by evening, there was still no clear-cut winner. both sides were badly bloodied.