The firing on Fort Sumter inspired thousands of free African Americans to volunteer their services for the Union cause. But even as the Southern states seceded from the Union and mustered armies to the fronts, a federal statute dating to 1792 prohibited blacks from service in the United States military.

The Lincoln administration feared dire consequences if blacks were allowed to fight for the Union. It was thought that blacks would make poor and undisciplined soldiers, and that integration would anger whites in the military. In addition, the administration believed that recruiting blacks would turn Northern civilians against the war and prompt the stillundecided border states to join the Confederacy.

As the war continued, however, and it grew apparent that the Union army would win no easy victory, more manpower was needed. On September 22, 1862, President Lincoln issued a preliminary Emancipation Proclamation...