Slave states and free states
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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.