Emboldened by Second Bull Run, Confederate forces launched their first invasion of the North in the Maryland Campaign during which Lee led 45,000 Army of Northern Virginia troops across the Potomac River into Maryland on September 5. Lincoln then restored Pope's troops to McClellan, and McClellan and Lee clashed in the Battle of Antietam near Sharpsburg, Maryland, on September 17, 1862, which proved the bloodiest single day in both the Civil War and US military history. Lee's army retreated to Virginia before McClellan could destroy it, leading the Battle of Antietam to be widely viewed as a Union victory since it halted Lee's invasion of the North and provided an opportunity for Lincoln to issue the Emancipation Proclamation, which he issued as an executive order on January 1, 1863.