In the history of the United States, 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, in the 17th century, slavery was established in a number of English overseas possessions. In the 18th century, it existed in all the British colonies of North America. In 1776, slavery was legal throughout the Thirteen Colonies; starting with Pennsylvania in 1780, about half the states abolished slavery during the Revolutionary War or in the first decades of the new country. Slavery became a divisive issue; it was a major issue during the writing of the U.S. Constitution, and slavery was the primary cause of the American Civil War. Just prior to the Civil War, there were 19 free states and 15 slave states. The Thirteenth Amendment to the United States Constitution, ratified in December of 1865, abolished slavery throughout the United States.

Historically, slavery in Eastern North America in the 17th and 18th century began in the English colonies. Slavery in the colonial US was established in each of the Thirteen Colonies. During British colonization, the number of people in slavery 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 in 1780, New Hampshire and Massachusetts in 1783, and Connecticut and Rhode Island in 1784. The Republic of Vermont had abolished 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 Americas.^[3] 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,^[2][4] although there were still hundreds of slaves in Northern "free" states as late as the 1840 census (see Slavery in the United States#Abolitionism in the North).

In the South, Kentucky was created a slave state from Virginia (1792), and Tennessee was created 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. Beginning with the Missouri Compromise of 1820, the dividing line between the slave and free states was called the Mason-Dixon line (between Maryland and Pennsylvania).

The 1787 Constitutional Convention debated slavery, and for a time slavery was a major impediment to passage of the new constitution. As a compromise, the institution was acknowledged though never mentioned directly in the constitution, as in the case of the Fugitive Slave Clause. The Constitution prohibited ending slave importation, but in a compromise, the prohibition would not go into effect for twenty years. Legislation ending it passed easily in 1807, effective in 1808. However, the lack of imports spurred a compensatory expansion in the domestic (internal) commerce in slaves, which remained legal under federal law until 1865.

The Northwest Ordinance of 1787, passed just before the U.S. Constitution was ratified, had prohibited slavery in the federal Northwest Territory. The southern boundary of the territory was the Ohio River, which was regarded as a westward extension of the Mason-Dixon line. The territory was generally settled by New Englanders and American Revolutionary War veterans granted land there. The 6 states created from the territory were all free states: Ohio (1803), Indiana (1816), Illinois (1818), Michigan (1837), Wisconsin (1848), and Minnesota (1858).

During the War of 1812, the British accepted as free all slaves who came into their hands, with no conditions as to military service such as had been made in Dunmore's Proclamation in the Revolutionary War. By the end of the War of 1812, the momentum for antislavery reform, state by state, appeared to run out of steam, with half of the states having already abolished slavery (Northeast), prohibited from the start (Midwest) or committed to eliminating slavery, and half committed to continuing the institution indefinitely (South).

The potential for political conflict over slavery at a federal level made politicians concerned about the balance of power in the United States Senate, where each State was represented by two Senators. With an equal number of slave states and free states, the Senate was equally divided on issues important to the South. As the population of the free states began to outstrip the population of the slave states, leading to control of the House of Representatives by free states, the Senate became the preoccupation of slave-state politicians interested in maintaining a Congressional veto over federal policy in regard to slavery and other issues important to the South. As a result of this preoccupation, slave states and free states were often admitted into the Union in opposite pairs to maintain the existing Senate balance between slave and free states.

Controversy over whether Missouri should be admitted as a slave state resulted in the Missouri Compromise of 1820, which specified that Louisiana Purchase territory north of latitude 36° 30', which described most of Missouri's southern boundary, would be organized as free states and territory south of that line would be reserved for organization as slave states. As part of the compromise, the admission of Maine (1820) as a free state was secured to balance Missouri's admission as a slave state (1820).

The admission of Texas (1845) and the acquisition of the vast new Mexican Cession territories (1848), after the Mexican–American War, created further North-South conflict. Although the settled portion of Texas was an area rich in cotton plantations and dependent on slave labor, the territory acquired in the Mountain West did not seem hospitable to cotton or slavery.

As part of the Compromise of 1850, California was admitted as a free state without a slave state pair. To avoid creating a free state majority in the Senate, California agreed to send one pro-slavery and one anti-slavery senator to Congress.

The difficulty of identifying territory that could be organized into additional slave states stalled the process of opening the western territories to settlement, while slave state politicians sought a solution, with efforts being made to acquire Cuba (see: Ostend Manifesto, 1852) and to annex Nicaragua (see: Walker affair, 1856–57), both to be slave states.

In 1854, the Missouri Compromise of 1820 was superseded by the Kansas–Nebraska Act, which allowed white male settlers in the new territories to determine through popular sovereignty whether they would allow slavery within each territory. The result was that pro- and anti-slavery elements flooded into Kansas with the goal of voting slavery up or down, leading to bloody fighting.^[5] An effort was initiated to organize Kansas for admission as a slave state, paired with Minnesota, but the admission of Kansas as a slave state was blocked because of questions over the legitimacy of its slave state constitution. Anti-slavery settlers in "Bleeding Kansas" in the 1850s were called Free-Staters and Free-Soilers, because they fought (successfully) to include Kansas in the Union as a free state in 1861.

When the admission of Minnesota proceeded unimpeded in 1858, the balance in the Senate was lost; a loss that was compounded by the subsequent admission of Oregon as a free state in 1859.

Before 1812, the concern about balancing slave-states and free states was not profound. The following table shows the slave and free states as of 1812. The year column is the year the state ratified the US Constitution or was admitted to the Union:

From 1812 through 1850, maintaining the balance of free and slave state votes in the Senate was considered of paramount importance if the Union were to be preserved, and states were typically admitted in pairs:

The balance was maintained until 1850:

The Civil War (1861-1865) disrupted and eventually ended slavery:

During the Civil War, a Unionist government in Wheeling, Virginia, presented a statehood bill to Congress in order to create a new state from 48 counties in western Virginia. The new state would eventually incorporate 50 counties. The issue of slavery in the new state delayed approval of the bill. In the Senate Charles Sumner objected to the admission of a new slave state, while Benjamin Wade defended statehood as long as a gradual emancipation clause would be included in the new state constitution.^[6] Two senators represented the Unionist Virginia government, John S. Carlile and Waitman T. Willey. Senator Carlile objected that Congress had no right to impose emancipation on West Virginia, while Willey proposed a compromise amendment to the state constitution for gradual abolition. Sumner attempted to add his own amendment to the bill, which was defeated, and the statehood bill passed both houses of Congress with the addition of what became known as the Willey Amendment. President Lincoln signed the bill on December 31, 1862. Voters in western Virginia approved the Willey Amendment on March 26, 1863.^[7]

President Lincoln had issued the Emancipation Proclamation on January 1, 1863, which exempted from emancipation the border states (four slave states loyal to the Union) as well as some territories occupied by Union forces within Confederate states. Two additional counties were added to West Virginia in late 1863, Berkeley and Jefferson. The slaves in Berkeley were also under exemption but not those in Jefferson County. As of the census of 1860, the 49 exempted counties held some 6000 slaves over 21 years of age who would not have been emancipated, about 40% of the total slave population.^[8] The terms of the Willey Amendment only freed children, at birth or as they came of age, and prohibited the importation of slaves.^[9]

West Virginia became the 35th state on June 20, 1863, and the last slave state admitted to the Union.^[10][11][12] Eighteen months later, the West Virginia legislature completely abolished slavery,^[13] and also ratified the 13th Amendment on February 3, 1865.

At the start of the Civil War, there were 34 states in the United States, 15 of which were slave states. Eleven of these slave states, after conventions devoted to the topic, issued declarations of secession from the United States and created the Confederate States of America and were represented in the Confederate Congress.^[14][15] The slave states that stayed in the Union, Maryland, Missouri, Delaware, and Kentucky (called border states) remained seated in the U.S. Congress. By the time the Emancipation Proclamation was issued in 1863, Tennessee was already under Union control. Accordingly, the Proclamation applied only to the 10 remaining Confederate states. Abolition of slavery, as mandated by the Thirteenth Amendment, was a requirement for readmission of the conquered Confederate states.

The border states of Maryland (November 1864)^[16] and Missouri (January 1865),^[17] one of the Confederate states, Tennessee (January 1865),^[18] and the new state of West Virginia, separated from Virginia in 1863 over the issue of slavery,^[19] abolished slavery just prior to the end of the Civil War in May, 1865. The U.S. Congress, after the departure of the dominant, powerful Southern contingent in 1861, was firmly abolitionist: slavery in the District of Columbia, which the Southern contingent had insisted on retaining, was abolished in 1862.^[20] However, slavery persisted in Delaware,^[21] Kentucky,^[22] and 10 of the 11 of the former Confederate states, until the Thirteenth Amendment to the United States Constitution abolished slavery throughout the United States on December 6, 1865, ending the distinction between slave and free states.^[23]