Slavery in the United States was the legal institution that existed in the United States of America in the 17th to 19th centuries. Slavery had been practiced in British North America from early colonial days, and was recognized in the Thirteen Colonies at the time of the United States' Declaration of Independence in 1776. After the Revolutionary War, abolitionist sentiment gradually spread in the Northern states, while the rapid expansion of the cotton industry from 1800 led to the Southern states strongly identifying with slavery, and attempting to extend it into the new Western territories. The United States was polarized by slavery into slave and free states along the Mason-Dixon Line, which separated Maryland and Pennsylvania.
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