in U.S. history, a controversial political doctrine that the people of federal territories should decide for themselves whether their territories would enter the Union as free or slave states. Its enemies, especially in New England, called it “squatter sovereignty.”

It was first applied in organizing the Utah and New Mexico territories in 1850; its most crucial application came with the passage of Senator Stephen A. Douglas' Kansas-Nebraska Act of 1854, which repealed the prohibition of slavery north of latitude 36°30′ (established in the Missouri Compromise of 1820 (Missouri Compromise)). The violent struggle that followed for control of the Kansas Territory (see Bleeding Kansas) illustrated the failure of popular sovereignty as a possible ground for agreement between proslavery and antislavery factions in the country. See also Dred Scott decision.