This map, published by the US Coast Guard, shows the percentage of slaves in the population in each county of the slave-holding states in 1860. The highest percentages lie along the Mississippi River, in the “Black Belt” of Alabama, and coastal South Carolina, all of which were centers of agricultural production (cotton and rice) in the United States. E. Hergesheimer (cartographer), Th. Leonhardt (engraver), Map Showing the Distribution of the Slave Population of the Southern States of the United States Compiled from the Census of 1860, c. 1861. Wikimedia.

Slavery’s history stretched back to antiquity. Prior to the American Revolution, nearly everyone in the world accepted it as a natural part of life. English colonies north and south relied on enslaved workers who grew tobacco, harvested indigo and sugar, and worked in ports. They generated tremendous wealth for the British crown. That wealth and luxury fostered seemingly limitless opportunities, and inspired seemingly boundless imaginations. Enslaved workers also helped give rise to revolutionary new ideals, ideals that in time became the ideological foundations of the sectional crisis. English political theorists, in particular, began to re-think natural law justifications for slavery. They rejected the longstanding idea that slavery was a condition that naturally suited some people. A new transatlantic antislavery movement began to argue that freedom was the natural condition of man was freedom.

Revolutionaries seized onto these ideas to stunning effect in the late eighteenth century. In the United States, France, and Haiti, revolutionaries began the work of splintering the old order. Each revolution seemed to radicalize the next. Bolder and more expansive declarations of equality and freedom followed one after the other. Revolutionaries in the United States declared, “All men are created equal,” in the 1770s. French visionaries issued the “Declaration of Rights and Man and Citizen” by 1789.  But the most startling development came in 1803. A revolution led by the island’s rebellious slaves turned France’s most valuable sugar colony into an independent country administered by the formerly enslaved.

The Haitian Revolution marked an early origin of the sectional crisis. It helped splinter the Atlantic basin into clear zones of freedom and un-freedom, while in the process shattering the longstanding assumption that African slaves could not also be rulers. Despite the clear limitations of the American Revolution in attacking slavery, the era marked a powerful break in slavery’s history. Military service on behalf of both the English and the American army freed thousands of slaves. Many others simply used the turmoil of war to make their escape. As a result, free black communities emerged—communities that would continually reignite the antislavery struggle. For nearly a century, most white Americans were content to compromise over the issue of slavery, but the constant agitation of black Americans, both enslaved and free, kept the issue alive.

The national breakdown over slavery occurred over a long timeline and across a broad geography.  Debates over slavery in the American West proved especially important. As the United States pressed westward in its search for new land and resources after its victory in the Revolution, new questions arose as to whether those lands ought to be slave or free. The framers of the Constitution did a little, but not much, to help resolve these early questions. Article VI of the 1787 Northwest Ordinance banned slavery north and west of the Ohio River. Many whites took it to mean that the founders intended for slavery to die out, as why else would they prohibit its spread across such a huge swath of territory?

Debates over the framer’s intentions often led to confusion and bitter debate, but the actions of the new government left better clues as to what the new nation intended for slavery. Congress authorized the admission of Vermont (1791) and Kentucky (1792), with Vermont coming into the Union as a free state, and Kentucky coming in as a slave state. Though Americans at the time made relatively little of the balancing act suggested by the admission of a slave state and a free state, the pattern became increasingly important. By 1820, preserving the balance of free states and slave states would be seen as an issue of national security.

New pressures challenging the delicate balance again arose in the West.  The Louisiana Purchase of 1803 more than doubled the size of the United States. Questions immediately arose as to whether these lands would be made slave or free. Complicating matters further was the rapid expansion of plantation slavery fueled by the invention of the cotton gin in 1793. Yet even with the booming cotton economy, many Americans, including Thomas Jefferson, believed that slavery was a temporary institution and would soon die out. The Louisiana Purchase signaled the beginning of rising sectional feelings, but a truly sectional national debate did not yet emerge.

That debate, however, came quickly. Sectional differences tied to the expansion of plantation slavery in the West were especially important after 1803. The Ohio Valley became an early fault line in the coming sectional struggle. Kentucky and Tennessee emerged as slave states, while free states Ohio, Indiana (1816) and Illinois (1818) gained admission along the river’s northern banks. Borderland negotiations and accommodations along the Ohio River fostered a distinctive kind of white supremacy, as laws tried to keep blacks out of the West entirely. Ohio’s so-called “Black Laws,” of 1803 foreshadowed the exclusionary cultures of Indiana, Illinois, and several subsequent states of the Old Northwest and later, the Far West. These laws often banned African American voting, denied black Americans access to public schools, and made it impossible for non-whites to serve on juries and in local militias, among a host of other restrictions and obstacles.

The Missouri Territory, by far the largest section of the Louisiana Territory, marked a turning point in the sectional crisis. Saint Louis, a bustling Mississippi River town filled with powerful slave owners, loomed large as an important trade headquarters for networks in the northern Mississippi Valley and the Greater West. In 1817, eager to put questions of whether this territory would be slave or free to rest, Congress opened its debate over Missouri’s admission to the Union. Congressman James Tallmadge of New York stirred up the trouble by proposing laws that would gradually abolish slavery in the new state. Southern states responded with unanimous outrage, and the nation shuddered at an undeniable sectional controversy.

Congress reached a “compromise” on Missouri’s admission, largely through the work of Kentuckian Henry Clay. Maine would be admitted to the Union as a free state. In exchange, Missouri would come into the Union as a slave state. Legislators sought to prevent future conflicts by making Missouri’s southern border at 36° 30′ the new dividing line between slavery and freedom in the Louisiana Purchase lands. South of that line, running east from Missouri to the western edge of the Louisiana Purchase lands (near the present-day Texas panhandle) slavery could expand. North of it, encompassing what in 1820 was still “unorganized territory,” there would be no slavery.

The Missouri Compromise marked a major turning point in America’s sectional crisis because it exposed to the public just how divisive the slavery issue had grown. The debate filled newspapers, speeches, and Congressional records. Anti-slavery and pro-slavery positions from that point forward repeatedly returned to points made during the Missouri debates. Legislators battled for weeks over whether the Constitutional framers intended slavery’s expansion or not, and these contests left deep scars. Even seemingly simple and straightforward phrases like “All Men Are Created Equal” were hotly contested all over again. Questions over the expansion of slavery remained open, but nearly all Americans concluded that the Constitution protected slavery where it already existed.

Southerners were not yet advancing arguments that said slavery was a positive good, but they did insist during the Missouri Debate that the framers supported slavery and wanted to see it expand. In Article 1, Section 2, for example, the Constitution enabled representation in the South to be based on rules defining enslaved people as 3/5 of a voter, meaning southern white men would be overrepresented in Congress. The Constitution also stipulated that Congress could not interfere with the slave trade before 1808, and enabled Congress to draft fugitive slave laws.

Antislavery participants in the Missouri debate argued that the framers never intended slavery to survive the Revolution and in fact hoped it would disappear through peaceful means. The framers of the Constitution never used the word “slave.” Slaves were referred to as “persons held in service,” perhaps referring to English common law precedents that questioned the legitimacy of “property in man.” Anti-slavery arguers also pointed out that while the Congress could not pass a law limiting the slave trade by 1808, the framers had also recognized the flip side of the debate and had thus opened the door to legislating the slave trade’s end once the deadline arrived. Language in the Tenth Amendment, they claimed, also said slavery could be banned in the territories. Finally, they pointed to the due process clause of the Fifth Amendment, which said that property could be seized through appropriate legislation. The bruising Missouri debates ultimately transcended arguments about the Constitution. They became an all-encompassing referendum on the American past, present, and future.

Despite the furor, debates over slavery unleashed during the Missouri Compromise did not yet develop into hardened defenses of either slave or free labor as positive good. Those would come in the coming decades, but in the meantime the uneasy consensus forged by the Missouri Debate managed to bring a measure of calm.

The Missouri debate had also deeply troubled the nation’s African Americans and Native Americans. By the time of the Missouri compromise debate, both groups saw that whites never intended them to be citizens of the United States. In fact, the debates over Missouri’s admission had offered the first sustained debate on the question of black citizenship, as Missouri’s State Constitution wanted to impose a hard ban on any future black migrants. Legislators ultimately agreed that this hard ban violated the Constitution, but reaffirmed Missouri’s ability to deny citizenship to African Americans. Americans by 1820 had endured a broad challenge, not only to their cherished ideals but also more fundamentally to their conceptions of self.