Enforced servitude in a society which acknowledges property rights over human beings. Forced labor. A violation of basic human rights, condemned by the United Nations.
In ancient times slaves were taken in war. It happened a lot to the Slavic peoples in the Dark Ages. Arguably the worst excess to date occured during the Slave Trade when African people were exported to the U.S.. This practice ended in 1807, but slavery itself did not end in the U.S. until the end of the American Civil War.
Speaking in Springfield, Illinois in October 1854, Abraham Lincoln said1 "... my ancient faith teaches me that 'all men are created equal' and that there can be no moral right in connection with one man's making a slave of another." and that "Slavery is founded on the selfishness of man's nature--opposition to it on his love of justice." On the strength of this argument Lincoln would ascend to the presidency without garnering a single Southern electoral college vote, and thus the stage for Southern secession and the American Civil War was set.
- Reported in Shelby Foote's The Civil War: A Narrative, Vol. 1, pg. 27