An animation showing the free/slave status of U.S. states and territories, 1789-1861 (see also: separate yearly maps below). The American Civil War began in 1861. The 13th Amendment, effective December 1865, abolished slavery in thje U.S..

In the history of the United States of America, a slave state was a U.S. state in which the practice of slavery was legal, and a free state was one in which slavery was prohibited or being legally phased out. Historically, all the British colonies had practiced slavery. The division between slave and free states began during the American Revolution (1775–1783). Slavery became a divisive issue and was the primary cause of the American Civil War. The Thirteenth Amendment to the United States Constitution, ratified in 1865, abolished slavery throughout the United States, and the distinction between free and slave states ended.

Slavery was legal and practiced in each of the Thirteen Colonies that became the US. During British colonization, the enslaved population expanded, primarily drawn from the Atlantic slave trade.[1] Organized political and social movements to end slavery began in the mid-18th century.[2] The sentiments of the American Revolution (1775-1783) and the equality evoked by the Declaration of Independence stood in contrast to the status of most black Americans. Despite this, thousands of black Americans fought against the British in hopes of a new order. Thousands also joined the British army, encouraged by British offers of freedom in exchange for military service.[2] (See Black Patriot and Black Loyalist).

In the 1770s, blacks throughout New England began sending petitions to northern legislatures demanding freedom. Five of the Northern self-declared states adopted policies to at least gradually abolish slavery: Pennsylvania (1780), New Hampshire and Massachusetts (1783), Connecticut and Rhode Island (1784). Vermont had abolished slavery in 1777, while it was still independent, and when it joined the United States as the 14th state in 1791, it was the first