On September 22, Abraham Lincoln issued the most important document of his Presidency: the preliminary Emancipation Proclamation. It promised that on January 1, 1863, those slaves living in unconquered Confederate states would be declared free.

The proclamation came as a surprise. Until then Lincoln had maintained that the Civil War was being fought solely to preserve the Union. Not that he believed in slavery. He had wanted to abolish it but feared doing so would be unconstitutional. In 1862, however, Lincoln read William Whiting’s The War Powers of the President , and it probably convinced him that as Commander in Chief he had the right to liberate the enemy’s slaves—but not those slaves in states already controlled by the Union. To have freed the latter would have dropped him into the “boundless field of absolutism,” Lincoln said. “The original proclamation has no constitutional or legal justification, except as a military measure.”

The proclamation changed the character of the war and profoundly affected its outcome. Sympathetic to the South, the governments of England and France had been waiting for a string of Southern victories before considering the recognition of the Confederacy as a nation. After the Emancipation Proclamation, however, the North was seen as fighting not just secessionists but slavery as well. The people of Europe would never tolerate their governments’ opposing such a cause. Although Lincoln didn’t know it, his action extinguished the South’s great hope of receiving military aid from Europe.

In the North, fighting to save the Union had been a somewhat cold and abstract purpose, but men’s hearts rose at the thought of fighting for human freedom. True, there were many who were outraged at spilling Northern blood to free Southern blacks. Nevertheless, the Emancipation Proclamation ennobled the conflict in a way that would help the North endure the years to come. “The act makes clear that the lives of our heroes have not been sacrificed in vain. It makes a victory of our defeats,” wrote Ralph Waldo Emerson at the time. “Our hurts are healed.…With a victory like this, we can stand many disasters.”