Missouri Compromise Finalizes Slave State Expansion After Racial Exclusion Crisis

Missouri became the 24th state on August 10, 1821, after Congress resolved a constitutional crisis over the state’s attempt to exclude free Black citizens. The original Missouri Compromise of March 1820 had admitted Missouri as a slave state paired with Maine as a free state, drawing a line at 36°30′ to prohibit slavery in remaining Louisiana Purchase territories. However, when Missouri’s constitutional convention empowered the legislature to bar free Blacks and mulattoes from entering the state, northern congressmen objected that this violated the Constitution’s privileges and immunities clause. Henry Clay engineered the Second Missouri Compromise requiring Missouri to pledge never to interpret the exclusionary clause to abridge citizens’ constitutional rights—though this proved meaningless in practice.

The compromise exposed deep anxieties about the growing “Slave Power” in federal institutions. Representative Daniel Pope Cook of Illinois warned that slaveholders would never honor restrictions on slavery’s expansion. When South Carolina’s William Lowndes “smiled and shook his head” at whether Congress could make such restrictions permanent, Cook exclaimed, “Away with your compromise. Let Missouri in, and the predominance of slave influence is settled, and the whole country will be overrun with it.” His prediction proved prescient. Thomas Jefferson called the controversy “a fire bell in the night” that filled him with terror, recognizing it as “the knell of the Union.”

The compromise established a pattern of constitutional manipulation that would define antebellum politics. Clay used aggressive parliamentary tactics to force passage, including “the neatest and cleverest parliamentary trick ever sprung in the House”—declaring a reconsideration motion out of order, then immediately signing and forwarding the bill before opponents could respond. This set a precedent for procedural manipulation in service of slavery’s expansion. The arrangement would hold for three decades until the Kansas-Nebraska Act of 1854 effectively repealed it, and the Supreme Court declared it unconstitutional in Dred Scott v. Sandford (1857), both accelerating the path to Civil War.