Excerpts from Books and Wikipedia
"In the summer of 1862, the Confederacy launched an offensive from Virginia under the command of General Robert E Lee, who moved north to cross the Potomac River into Maryland. He intended to invade Pennsylvania, but his advance ended with the bloodiest battle of the entire war." [Furtado: 1001 Days] "McClellan had halted Lee's invasion of Maryland, but his army had suffered heavier losses and Lee was able to withdraw his army back to Virginia without interference. McClellan's refusal to pursue Lee's army led to his removal from command by President Abraham Lincoln in November. Although the battle was tactically inconclusive, the Confederate troops had withdrawn first from the battlefield and abandoned their invasion, making it a Union strategic victory. It was enough of a victory to give Lincoln the confidence to announce his Emancipation Proclamation, which by freeing more than 3.5 million slaves in the Confederate states (but not in Union slave states), began the process of emancipation of all remaining persons legally considered slaves within the United States and in doing so, discouraged the British and French governments, which were strongly opposed to slavery and had in fact abolished slavery prior to the American Civil War, from recognizing the Confederacy." [Wikipedia]