Slave states and free states

In the United States before 1865, a free state was a state in which slavery and the internal or domestic slave trade were prohibited, while a slave state was one in which they were legal. 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 Clause of the U.S. Constitution, as implemented by the Fugitive Slave Act of 1793 and the Fugitive Slave Act of 1850, provided that a slave did not become free by entering a free state and must be returned to their owner. Enforcement of these laws became one of the controversies that arose between slave and free states.

By the 18th century, slavery was legal throughout the Thirteen Colonies, but at the time of the American Revolution, rebel colonies started to abolish the practice. Pennsylvania abolished slavery in 1780, and about half the states had abolished slavery by the end of the Revolutionary War or in the first decades of the new country's existence, although, depending on the jurisdiction, this did not mean that all slaves became immediately free due to gradual abolition. 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 it 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 states, and the Thirteenth Amendment to the United States Constitution, ratified in December 1865, abolished chattel slavery throughout the United States.

Slavery was established as a legal institution in each of the Thirteen Colonies, starting in 1619 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 abolish slavery began in the mid-18th century. The sentiments of the American Revolution and the promise of equalityevoked by the Declaration of Independence stood in contrast to the status of most Black people, 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 Northern 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.

The 1787 Constitutional Convention debated slavery, and for a time slavery was a major impediment to passage of the new constitution. As a compromise, slavery was acknowledged but never mentioned explicitly in the Constitution. The Fugitive Slave Clause, Article 4, section 2, clause 3, for example, refers to a "Person held to Service or Labor." In addition, Article 1, section 9, clause 1 of the Constitution prohibited Congress from abolishing the importation of slaves, but in a compromise, the prohibition could be lifted by Congress in 20 years, and slaves were referred to as "Persons." The Act Prohibiting Importation of Slaves passed easily in 1807 and took effect on January 1, 1808. However, the ban on importation spurred an expansion in the domestic slave trade, which remained legal until slavery was banned entirely in 1865 by the 13th Amendment.

In the late 1850s, several Southern states launched an unsuccessful campaign to resume the international slave trade, in order to restock their slave populations, but this met with strong opposition. However, a large natural increase in the slave population occurred throughout the late 18th and 19th centuries, while some illegal smuggling of African slaves continued via Spanish Cuba.