Which best states why the battle of antietam was considered a turning point in the civil war?

The Army of the Potomac, under the command of George McClellan, mounted a series of powerful assaults against Robert E. Lee’s forces near Sharpsburg, Maryland, on September 17, 1862. The morning assault and vicious Confederate counterattacks swept back and forth through Miller’s Cornfield and the West Woods. Later, towards the center of the battlefield, Union assaults against the Sunken Road pierced the Confederate center after a terrible struggle. Late in the day, the third and final major assault by the Union army pushed over a bullet-strewn stone bridge at Antietam Creek. Just as the Federal forces began to collapse the Confederate right, the timely arrival of A.P. Hill’s division from Harpers Ferry helped to drive the Army of the Potomac back once more. The bloodiest single day in American military history ended in a draw, but the Confederate retreat gave Abraham Lincoln the “victory” he desired before issuing the Emancipation Proclamation.

Use the formula: (n-2)×180, where n is the number of sides(18-2)×180= 28800, now this is the full exterior angle of 18-gon so you divide it by 18.28800÷18=160