Why did the expansion of slavery into western territories cause conflict before the American Civil War?

The expansion of slavery into western territories was a central driver of sectional conflict in the United States, primarily because it threatened the political balance of power between the North and the South.12

Political Balance and Sectional Tension

The U.S. Senate relied on an equal number of free and slave states to maintain a legislative stalemate. When new territories sought statehood, both regions feared that the admission of a new state would tip the balance in favor of the other. Southerners argued that they had a constitutional right to take enslaved people into these territories, while Northerners increasingly sought to prevent the spread of slavery, viewing it as a moral issue and a threat to free labor.12

Key Legislative Conflicts

• Missouri Compromise (1820): Attempted to maintain balance by admitting Maine as a free state and Missouri as a slave state, while prohibiting slavery north of latitude 36°30′.12
• Wilmot Proviso (1846): A failed proposal to ban slavery in territory acquired from the Mexican-American War, which galvanized antislavery sentiment under the banner of "free soil".13
• Kansas-Nebraska Act (1854): Repealed the Missouri Compromise by allowing popular sovereignty, leading to violent conflict known as "Bleeding Kansas".145
• Dred Scott Decision (1857): The Supreme Court ruled that Congress could not prohibit slavery in the territories, effectively voiding previous compromises and intensifying the national crisis.16