Slave rebellions, by Gabriel Prosser (1800), Denmark Vesey (1822), Nat Turner (1831), and most famously by John Brown (1859), caused fear in the white South, which imposed stricter oversight of slaves and reduced the rights of free blacks. The Fugitive Slave Act of 1850 required the states to cooperate with slave owners when attempting to recover escaped slaves, which outraged Northerners. Formerly, an escaped slave that reached a non-slave state was presumed to have attained sanctuary and freedom under the Missouri Compromise. The Supreme Court's 1857 decision in "Dred Scott v. Sandford" ruled that the Missouri Compromise was unconstitutional; angry Republicans said this decision threatened to make slavery a national institution.