How did a war to preserve the union became a war to end slavery?

It became apparent to Abraham Lincoln in the middle of 1862 that the original aim of quashing the rebellion were ineffective due to military reverses, specificaly the failure of McClellan's effort to capture Richmond in the Peninsular Campaign. This feat of Southern arms against the Army of the Potomac had stiffened Confederate resolve, and more importantly had threatened British recognition of the CSA. It was therefore necessary for Lincoln to add the moral stigma of slavery to the offensive in order to 1)sway British opinion against diplomatically recognizing the South and 2) to energize the slave population into both passive (through refusing to work) and active (the raising of "colored" regiments ) resistance against the Confederate rebels. Lincoln waited for 2 days after the Battle of Antietam (Sharpsburg) to release the Emancipation Proclamation, thereby announcing the new war aims in September of 1862.