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Slavery in Great Britain had been practiced but never authorized by statute. In 1772 it was made partially unenforceable at common law in Great Britain by a court decision, but this did not stem the tide of British participation in the international slave trade. That would continue until 1807 through the work of William Wilberforce and the passage of the Slave Trade Act. Slavery continued to flourish in most of Britain's colonies, including in the American colonies.
In early 1775 Lord Dunmore, the royal governor of Virginia, declared his intention to free slaves owned by Patriots. On November 7, 1775, he issued what became known as Lord Dunmore's Proclamation, which declared martial law and promised freedom for any slaves of American patriots who would leave their masters and join the royal forces. Slaves owned by Loyalist masters, however, were unaffected by Dunmore's Proclamation. About 1500 slaves owned by Patriots escaped and joined Dunmore's forces. Three hundred of these freed slaves made it to freedom in Britain. Many others died from disease. A large number of slaves used the chaos caused by war to escape their plantations and fade into cities or woods. Throughout the South, losses of slaves were high, with many due to escapes. Slaves also escaped throughout New England and the mid-Atlantic, joining the British who had occupied New York.
In the closing months of the war, the British evacuated 20,000 freedmen from major coastal cities. More than 3,000 were taken for resettlement in Nova Scotia, where they were registered as Black Loyalists and eventually granted land. They transported others to the Caribbean islands, and some to England. The British also transported Loyalists and their slaves, to the Caribbean, and to Nova Scotia.
For those slaves and free former slaves who fought on the side of rebels during the Revolutionary War, George Washington authorized slaves to be freed if they had fought with the American Continental Army. During the course of the war, about one fifth of the northern army was composed of former slaves and freedmen.
In 1777, the colonies outlawed the importation of slaves state by state. They all acted to end the international trade but it was later reopened in South Carolina and Georgia. In 1807 Congress acted on President Jefferson's advice and made importing slaves from abroad a federal crime. During his presidency, Jefferson allowed the diffusion of slavery into the Louisiana Territory hoping to prevent South Carolina's secession. In 1804, in a compromise on the slavery issue, Jefferson and Congress banned domestic slave trafficking for one year into the Louisiana Territory. In 1806 he called for anti-slavery legislation terminating the import or export of slaves. Congress passed the law in 1807, taking effect in 1818.
The Constitution of the United States had come into effect in 1789 and included several provisions regarding slavery. Section 9 of Article I forbade the Federal government from banning the importation of slaves before January 1, 1808. As a protection for slavery, the delegates approved Section 2 of Article IV, which prohibited states from freeing slaves who fled to them from another state, and required the return of chattel property to owners.
In a section negotiated by James Madison of Virginia, Section 2 of Article I designated "other persons" (namely, slaves) to be added to the total of the state's free population, at the rate of three-fifths of their total number, to establish the state's official population for the purposes of apportionment of Congressional representation and federal taxation. The protections afforded slavery in the Constitution disproportionately strengthened the political power of Southern representatives, as three-fifths of the (non-voting) slave population was counted for Congressional apportionment.
In the 72 years between the election of George Washington and the election of Abraham Lincoln, 50 of those years [had] a slaveholder as president of the United States, and, for that whole period of time, and every president elected to a second term was a slaveholder. This increased the power of southern states in Congress strongly influenced national policies and legislation, and the planter elite dominated the southern Congressional delegations.
In the north however, it was a different story. Beginning during the revolution and in the first two decades of the postwar era, every state in the North abolished slavery, ending with New Jersey in 1804. In Massachusetts, slavery was successfully challenged in court in 1783. Most northern states passed legislation for gradual abolition, first freeing children born to slave mothers (and requiring them to serve lengthy indentures to their mother's masters, often into their 20s as young adults). New York did not fully free its last ex-slaves until 1827, Rhode Island had seven slaves still listed in the 1840 census. Pennsylvania's last ex-slaves were freed in 1847, Connecticut's in 1848, and New Hampshire and New Jersey in 1865.
None of the Southern states abolished slavery. Some individual slaveholders in the South freed some or all of their slaves in their wills. Methodist, Quaker and Baptist preachers traveled in the South, appealing to slaveholders to manumit their slaves. By 1810, the number and proportion of freed former slabes in the population of the United States had risen dramatically. Most resided in the North.
The Northwest Ordinance of 1787 prohibited slavery in the territories northwest of the Ohio River. Existing slaves in the Territory were not freed for years, although they could no longer be sold. Thomas Jefferson proposed in 1784 to end slavery in all the territories, but his bill failed to pass in the House of Representatives by one vote. The territories south of the Ohio River authorized slavery. Since Northerners were the majority of settlers in the westward movement into the Midwestern territory after the American Revolution, the states that were organized from these territories voted to prohibit slavery in their constitutions when they achieved statehood. These included Ohio in 1803, Indiana in 1816, and Illinois in 1818. An anti-slavery culture developed in these states.
The emancipation of slaves in the North led to the growth in the population of northern freedmen, from several hundreds in the 1770s to nearly 50,000 by 1810.