The correct answer is the Emancipation Proclamation.
This law, signed by President Abraham Lincoln, was to take effect on January 1st of 1863. This law was supposed to free slaves in the "rebellious states" meaning that slaves in the Confederacy would be free. Even though the Confederate states ignored this law since they considered themselves an independent and sovereign nation, one positive impact of this law was the allowing of African American men to join the armed forces. This lead to the development of all black units within the Union army.