In the United States before 1865, a slave state was a state in which slavery and the internal or domestic slave trade were legal, while a free state was one in which they were prohibited. Between 1812 and 1850, it was considered by the slave states to be politically imperative that the number of free states not exceed the number of slave states, so new states were admitted in slave–free pairs. There were, nonetheless, some slaves in most free states up to the 1840 census, and the Fugitive Slave Act of 1850 specifically stated that a slave did not become free by entering a free state.
Slavery in what would become the United States was established as part of European colonization. By the 18th century, slavery was legal throughout the Thirteen Colonies, after which rebel colonies started to abolish the practice. Pennsylvania abolished slavery in 1780, and about half of the states had abolished slavery by the end of the Revolutionary War or in the first decades of the new country, although this did not always mean that existing slaves became free. Vermont – having declared its independence from Britain in 1777 and thus not being one of the Thirteen Colonies – banned slavery in the same year, before being admitted as a state in 1791.
Slavery was a divisive issue in the United States. It was a major issue during the writing of the U.S. Constitution in 1787, the subject of political crises in the Missouri Compromise of 1820 and the Compromise of 1850, and was the primary cause of the American Civil War in 1861. Just before the Civil War, there were 19 free states and 15 slave states. The most recent free state, Kansas, had entered the Union after its own years-long bloody fight over slavery. During the war, slavery was abolished in some of the slave state jurisdictions, and the Thirteenth Amendment to the United States Constitution, ratified in December 1865, finally abolished slavery throughout the United States.
Early history
Slavery was established as a legal institution in each of the Thirteen Colonies, starting from 1619 onwards with the arrival of "twenty and odd" enslaved Africans in Virginia. Although indigenous peoples were also sold into slavery, the vast majority of the enslaved population consisted of Africans brought to the Americas via the Atlantic slave trade. Due to a lower prevalence of tropical diseases and better treatment, the enslaved population in the colonies had a higher life expectancy than in the West Indies and South America, leading to a rapid increase in population in the decades prior to the American Revolution. Organized political and social movements to end slavery began in the mid-18th century. The sentiments of the American Revolution and the promise of equality evoked by the Declaration of Independence stood in contrast to the status of most blacks, either free or enslaved, in the colonies. Despite this, thousands of black Americans fought for the Patriot cause for a combination of reasons. Thousands also joined the British, encouraged by offers of freedom such as the Philipsburg Proclamation.
In the 1770s, enslaved black people 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 in 1780, New Hampshire and Massachusetts in 1783, and Connecticut and Rhode Island in 1784. The Republic of Vermont had limited slavery in 1777, while it was still independent before it joined the United States as the 14th state in 1791. These state jurisdictions thus enacted the first abolition laws in the Atlantic World. By 1804 (including New York (1799) and New Jersey (1804)), all of the Northern states had abolished slavery or set measures in place to gradually abolish it, although there were still hundreds of ex-slaves working without pay as indentured servants in Northern states as late as the 1840 census (see Slavery in the United States#Abolitionism in the North).
In the South, Kentucky was created as a slave state from Virginia (1792), and Tennessee was created as a slave state from North Carolina (1796). By 1804, before the creation of new states from the federal western territories, the number of slave and free states was 8 each. By the time of Missouri Compromise of 1820, the dividing line between the slave and free states was called the Mason-Dixon line (between Maryland and Pennsylvania), with its westward extension being the Ohio River.