Missouri Compromise: An agreement reached in 1820 between pro-slavery and anti-slavery factions that allowed Missouri to enter the Union as a slave state while Maine entered as a free state, thus maintaining the balance between slave and free states.
Kansas-Nebraska Act: A law passed in 1854 that allowed popular sovereignty (the people's vote) to determine whether Kansas and Nebraska would be admitted as slave or free states, effectively repealing the ban on slavery in certain territories established by the Missouri Compromise.
Dred Scott v. Sandford: A landmark Supreme Court case in 1857 where it was ruled that enslaved African Americans were not citizens and therefore could not sue for their freedom; the decision also stated that Congress had no power to prohibit slavery in any territory.