The Emancipation Proclamation was issued on January 1, 1863, immediately after the Battle of Antietam. It did NOT, contrary to popular opinion, "free" slaves; in fact it not only excluded slaves in the so-called border states; it also excluded slaves in the South from those areas under Union Control. Its primary effect--and purpose--was to prevent European intervention in the War on the Southern side. By invading Maryland, General Robert E. Lee had hoped to gain recognition of the Confederacy by major European powers, primarily France and Great Britain, the latter of which already bought large amounts of southern cotton. By issuing the Proclamation, Lincoln transformed the purpose of the war into a war over the existence of slavery within the United States. Since Britain and France had previously abolished slavery, Lincoln created a moral dilemma for them--support the South and you are supporting slavery. This forever precluded the possibility of European intervention.
As noted, the Proclamation also turned the war into one over the issue of slavery. By so doing, Lincoln gave the North the advantage of occupying the moral high ground. However, its major impact on the war was to prevent European intervention in the war, which might have led to a different conclusion.