The Thirteenth Amendment, passed by Congress in 1865, says that

The Thirteenth Amendment says that, "Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction."

In other words, African Americans could no longer be held as slaves and were free to live their lives. The Thirteenth Amendment made slavery illegal in all states.