Nullification Crisis: A dispute between President Andrew Jackson and South Carolina over whether states had the right to reject federal laws they considered unconstitutional.
Kansas-Nebraska Act: An act passed by Congress in 1854 that repealed previous restrictions on slavery in newly organized territories, leading to violent conflicts between pro-slavery and anti-slavery settlers.
Emancipation Proclamation: An executive order issued by President Abraham Lincoln during the Civil War that declared all slaves in Confederate territory to be forever free.