This is stated in Section 1 of the Fourteenth Amendment to the United States Constitution. The Fourteenth Amendment was passed in order to guarantee that freed slaves would have basic rights of citizenship. This section of the amendment speeled out this protection. Over time, the protections of the Fourteenth Amendment have been extended to the states, and it has been used to overturn laws that violated the constitutional rights of citizens. This has been done largely under the "due process" and "equal protection" clauses of the Amendment, which protects all citizens from arbitrary acts of government as well as stipulating that all citizens ought to be equally protected by the laws.