The extension of slavery to new territories had been a subject of national political controversy since the Northwest Ordinance of 1787 prohibited slavery in the area now known as the Midwest. The Missouri Compromise of 1820 began a policy of admitting an equal number of slave and free states into the Union. But the Compromise of 1850 and the Kansas-Nebraska Act of 1854 (both grounded in the doctrine of popular sovereignty), along with the Supreme Court’s Dred Scott decision of 1857, opened all the territories to slavery.
