The bloodiest day in American history took place on the 75th anniversary of the signing of the Constitution. On September 17, 1862, Robert E. Lee's Confederate Army of Northern Virginia fought George McClellan's Union Army of the Potomac outside Sharpsburg along Antietam Creek. That day, nearly 25,000 would become casualties, and Lee's army would barely survive fighting the much bigger Northern army. Although the battle was tactically a draw, it resulted in forcing Lee's army out of Maryland and back into Virginia, making it a strategic victory for the North and an opportune time for President Abraham Lincoln to issue the Emancipation Proclamation, freeing all slaves in the rebellious states. For those reasons, Antietam is remembered as one of the major turning points of the Civil War, but it is often overlooked that the bloody battle only represented the climactic culmination of a 3 week campaign that saw George McClellan cautiously pull a fragmented Union army together and begin tracking Lee's army into Maryland. Sizing up McClellan, Lee had split his army up during its invasion, including sending Stonewall Jackson's men to Harpers Ferry, but the whole course of the campaign and possibly the war changed when the Union Army somehow found a copy of Lee's marching orders, telling them where the Confederate army would be and when. To Lee's surprise, McClellan's army began advancing far more rapidly, including attacking them at South Mountain before cornering them along Antietam Creek outside of Sharpsburg. The Army of the Potomac had pushed Robert E. Lee's army out of Maryland in September 1862 after the Battle of Antietam, but President Lincoln and his War Department wanted the army to continue going after the Army of Northern Virginia after it retreated back into Virginia. When George B. McClellan refused to do it, Lincoln fired him and installed Ambrose E. Burnside as the new commander. Burnside, who didn't believe himself capable of commanding the Army of the Potomac, only took the job because he was told Fighting Joe Hooker would get the spot if he refused. With Washington urging Burnside to advance against Lee, Burnside launched an ill fated operation across the Rappahannock River near Fredericksburg in December 1862. From December 12-13, Burnside struggled to get his army across the river while it was under fire from Confederates in Fredericksburg, and things only got worse when they did. Although the Union almost broke the Confederate lines in the south on December 13, they were ultimately repulsed, and the battle is mostly remembered for the piecemeal attacks the Union army made on heavily fortified positions Longstreet's men took up on Marye's Heights. As they threw themselves at Longstreet's heavily fortified position along the high ground, the Northern soldiers were mowed down again and again. General Longstreet compared the near continuous fall of soldiers on the battlefield to "the steady dripping of rain from the eaves of a house." During the battle, Lee turned to Longstreet and commented, "It is well that war is so terrible, otherwise we should grow too fond of it." As injured Northern soldiers lay freezing and dying on the field that night, the Northern Lights made a rare appearance. Southern soldiers interpreted it as a favorable omen from God and mentioned them frequently in their diaries, while Northern soldiers who saw something far less divine sparsely mentioned them. The following morning, Burnside extricated his army back behind the river, ending the fighting in 1862.