In American politics and Constitutional law states' rights are guaranteed by the Ninth Amendment to the United States Constitution, (i.e. the ninth article of the Bill of Rights). It is usually used to defend a state law that the Federal government seeks to override, or a perceived violation of the bounds of Federal authority.
The principle of federal rights over those rights held by the states was laid out by John Marshall, the first Chief Justice of the Supreme Court of the United States. In the seminal case of McCulloch v. Maryland, Marshall asserted, based on the supremacy clause of the United States Constitution, that the laws of the federal government were generally paramount over the laws of the separate state governments.
The principle of states' rights was the fulcrum on which many of the political battles preceding the American Civil War were balanced. Slave states asserted that they had the right to maintain social institutions—particularly slavery—in whichever way their state legislatures saw fit, while the federal government and Northerners tended to disagree. Confederate States also believed that states' rights authorized them to dissolve the Union. Again, the federal government disagreed.
During the civil rights movement of the 1950s and 1960s, states' rights again become strongly associated with Southern racial politics, with proponents of segregation and Jim Crow laws denouncing federal interference in these state-level policies.
The extent of states' rights remains a hotly-debated topic to this day. The use (or non-use) of the death penalty is currently decided by individual states. Other controversial subjects entering the states' rights debate include the right to legalize assisted suicide, the right to legalize gay marriage, and the right to legalize medical marijuana, the last of which is in direct contravention of current federal U.S. law.