The Fourteenth Amendment, ratified in 1868, was the one that clarified the requirements for being a U.S. citizen, as it establishes: "All persons born or naturalized in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the state wherein they reside."
This Amendment was the one that granted citizenship to African American and former slaves, for the first time. And it also guaranteed due process of law and equal protection of the laws to all citizens.