Answer:
The answer is President Lincoln's Emancipation Proclamation.
Step-by-step explanation:
Though "contraband" slaves had been declared free, Lincoln continued to insist that this was a war to save the Union, not to free slaves. But by 1862, Lincoln was considering emancipation as a necessary step toward winning the war. The South was using enslaved people to aid the war effort. Black men and women were forced to build fortifications, work as blacksmiths, nurses, boatmen, and laundresses, and to work in factories, hospitals, and armories. In the meantime, the North was refusing to accept the services of black volunteers and freed slaves, the very people who most wanted to defeat the slaveholders. In addition, several governments in Europe were considering recognizing the Confederacy and intervening in the Civil War.
On July 22, 1862, Lincoln showed a draft of the preliminary Emancipation Proclamation to his cabinet. It proposed to emancipate the slaves in all rebel areas on January 1, 1863. Secretary of State William H. Seward agreed with the proposal, but cautioned Lincoln to wait until the Union had a major victory before formally issuing the proclamation. Lincoln's chance came after the Union victory at the Battle of Antietam in September of 1862. He issued the preliminary Emancipation Proclamation on September 22. The proclamation warned the Confederate states to surrender by January 1, 1863, or their slaves would be freed.
Some people were critical of the proclamation for only freeing some of the slaves. Others, including Frederick Douglass, were jubilant. Douglass felt that it was the beginning of the end of slavery, and that it would act as a chain reaction within the Confederacy. Yet, he and others feared that Lincoln would give in to pressure from northern conservatives and would fail to keep his promise. Despite the opposition, however, the president remained firm. On January 1, 1863, he issued the final Emancipation Proclamation. With it he officially freed all slaves within the states or parts of states that were in rebellion and not in Union hands.