Slavery or involuntary servitude can be re-imposed as a punishment

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The Thirteenth Amendment to the United States Constitution abolished slavery and involuntary servitude, except as punishment for a crime. It was passed by the Senate on April 8, 1864, by the House on January 31, 1865, and adopted on December 6, 1865. On December 18, 1865, Secretary of State William H. Seward proclaimed its adoption. It was the first of the three Reconstruction Amendments adopted following the American Civil War.


Slavery had been tacitly protected in the original Constitution through clauses such as the Three-Fifths Compromise, by which three-fifths of the slave population was counted for representation in the United States House of Representatives. Though many slaves had been declared free by Lincoln's 1863 Emancipation Proclamation, their post-war status was uncertain. On April 8, 1864, the Senate passed an amendment to abolish slavery. After one unsuccessful vote and extensive legislative maneuvering by the Lincoln administration, the House followed suit on January 31, 1865. The measure was swiftly ratified by nearly all Union states, and by a sufficient number of border and "reconstructed" Southern states to cause it to be adopted before the end of the year.


Though the amendment formally abolished slavery throughout the United States, factors such as Black Codes, white supremacist violence, and selective enforcement of statutes continued to subject some black Americans to involuntary labor, particularly in the South. In contrast to the other Reconstruction Amendments, the Thirteenth Amendment was rarely cited in later case law, but has been used to strike down peonage and some race-based discrimination as "badges and incidents of slavery". While the Fourteenth and Fifteenth Amendments apply only to state actors, the Thirteenth applies also to private citizens. The amendment also enables Congress to pass laws against sex trafficking and other modern forms of slavery.

http://en.wikipedia.org/wiki/Thirteenth_Amendment_to_the_United_States_Constitution