Gideon Wells, Secretary of the Navy, was also in attendance that day [the cabinet meeting of September 22, 1862] and he too made extensive notes. The president, Welles recorded, "remarked that he had made a vow, a covenant, that if God gave us the victory in the approaching battle he would consider it an indication of Divine will, and that it was his duty to move forward in the cause of emancipation." Welles also captured Lincoln's memorable conclusion: "God had decided this question in favor of the slaves."
True to his word, Lincoln issued The Emancipation Proclamation on January 1, 1863. To the end of his days, he considered it his greatest act.
It was not a perfect document. Since it was a war act, it could only free slaves held in states then in rebellion against the United States. It did not apply to Union states. Lincoln did not have that authority. However, the act did free tens of thousands of slaves in "contraband camps" throughout Union-controlled portions of North and South Carolina, Georgia, Florida, Mississippi and Arkansas. Perhaps as important, the Proclamation transformed the purpose of the war. Prior to January 1, 1863, the war had been about preserving the union. Afterward, Union armies were transformed into armies of liberation.
Indeed, there is considerable question whether Lincoln had as much authority as he did exercise. It was the most audacious assertion of presidential war powers in American history. Yet the transformation of the war effort was profound and important. If the Union won, complete abolition would be inevitable.