States' rights is the political doctrine that the federal government should not have the power or authority to tell any individual state, or group of states, what to do. The idea of states' rights has most recently been important in the 1950's and 60's, when the states of the former Confederacy refused to implement federal Civil Rights legislation.
States' rights is also frequently cited as a major cause of the ( American ) Civil War. It has become popular to insist that the war wasn't about slavery, but about states' rights. This becomes a bit silly when carried too far, since the states' right the South wanted to preserve was the right to own slaves. It wasn't actually a states' rights issue before the Civil War because the North did not have the political wherewithal to abolish slavery, only to attempt to check its spread. There was no anti-slavery legislation to defy. Slavery was not abolished until during the war--with the Southern representatives and Senators absent, the North had a clear majority for its policies.
The states that did defy national legislation before the Civil war were Northern--the Northern states, more or less, refused to enforce the federal Fugitive Slave Act, which demanded the return of any slave who escaped to freedom in the North. Southerners have actually cited this as a cause of their defiance, 100 years later, of federally ordered desegregation--states' rights for states' rights.
