Kansas-Nebraska Act, 1854, a federal law that stirred bitter controversy over slavery. It created the Kansas and Nebraska territories. Under the Missouri Compromise of 1820, this area was closed to slavery. However, to the outrage of abolitionists and others opposed to the extension of slavery, the Kansas-Nebraska Act stated that the Missouri Compromise was no longer in effect. The act provided that slavery could exist in any state created in the territories if the voters so decided. This provision came to be known as “squatter sovereignty.” Senator Stephen A. Douglas of Illinois was sponsor of the bill.

The terms of the law led to a fierce struggle in Kansas between antislavery and proslavery forces. The antislavery people won, but not until the state became known as “bleeding Kansas” because of killings and other violence. The Republican party was largely an outgrowth of opposition to the Kansas-Nebraska law.