General Ambrose Burnside and his division were ordered to launch an attack from the lower part of a Rohrbach Bridge against the Confederate artillery on the hillside. The Confederates had an advantage as they were able to defend their position and use trees as covers while the Union had to use a narrow road to cross a bridge that was under fire from Confederate artillery. The Union tried two times to cross the bridge and failed both times until they regroup for a third attack. The 51st Pennsylvania and the 51st New York, commanded by Col. John Hartranft, was ordered to take the bridge and the highland. According to legend, the men asked if they would be rewarded with extra whiskey if they successfully dislodged the Confederate defenders. After two hours of heavy fighting and high casualties, the Union crossed the bridge but lost over 500 men in the process. This allowed the Union to drive the Confederates from their position and gave the Union control of the highlands.
As the Federal forces began to the collapse the Confederate right flank, the timely arrival of General AP Hill’s Division helped drive the Union forces back, and allow Lee to escape back to Virginia. The battle cost both sides 22,720 casualties and 3,650 dead. Antietam was the bloodiest one-day battle of the war and there was no clear victor. However, Lee's army was too devastated to continue an offensive in Northern territory which allowed the Union to claim a technical victory as Union generals never lost command of the field.
Antietam was the first major victory of the Army of the Potomac over Lee's Army of Northern Virginia. The Union victory at Antietam helped solidify Lincoln’s position in Washington because the President was able to issue the Preliminary Emancipation Proclamation which announced that the Union would consider all slaves in rebel territory free after the end of the year. The Emancipation Proclamation went into effect in January 1863, and although it only applied to areas in rebellion rather than lands Lincoln's armies controlled, the proclamation demonstrated that the issue of slavery was at the center of the war and signalled that Lincoln would no longer be content to return to the status quo as a nation divided half slave and half free
The Emancipation Proclamation also reduced the likelihood of European help for the South given the sentiment of most European leaders against chattel slavery after Britain outlawed the practice in their colonies. The Union's ability to stop Lee's offensive coupled with the President's decision to move closer to abolitionist Frederick Douglass's vision of a war for emancipation came at a time when several European powers were considering support for the Confederacy in order to preserve advantageous trade relationships with Southern ports and merchants.
Reflecting the historiographical shift towards interpreting military and social history simultaneously, leading historians such as James McPherson began interpreting the battle as a pivotal moment in the war for several reasons. Few Americans could comprehend the sheer devastation of Antietam where 6500 soldiers were mortally wounded in a single day and nearly twenty thousand were seriously wounded. And while Antietam marked the bloodiest day in American history, it marked a transition from the maneuvering of generals like McClellan to the total warfare and sustained campaigns led by future Union leaders. According to McPherson and many other historians, Antietam also marked a moral and political transition given Lincoln's decision to issue the Emancipation Proclamation in its wake.
Military historians are quick to balance this interpretation with the limits of what the Union achieved at Antietam. McPherson agrees with the general consensus of military historians who believe that had McClellan pressed his attack on Lee, he might have crushed the Confederate Army of Northern Virginia as Lee was forced to retreat and in a vulnerable position as his army needed to either cross the Potomac River to return to the South. However, McClellan decided to not attack the fleeing Confederate army, allowing Lee to cross the Potomac. Lincoln was furious about McClellan’s lack of aggressiveness and resolved to replace McClellan following the battle.