Nullification theory is the idea that states have the right to invalidate any federal law they believe is unconstitutional. This concept emerged in the early 19th century, particularly associated with the Kentucky and Virginia Resolutions of 1798, which argued that states could refuse to enforce federal laws. The theory gained prominence during the Nullification Crisis of the 1830s, when South Carolina attempted to nullify federal tariffs. The crisis highlighted tensions between state and federal authority, ultimately leading to a stronger assertion of federal power and the idea that states cannot unilaterally reject federal laws.