In the first half of the 1800s, there were many arguments about slavery in the United States. During this time, every time a free state (a state where slavery was illegal) was added into the country, a slave state (a state where slavery was allowed) had to be added too. The Missouri Compromise continued to let this happen. As a compromise to people arguing about new states being free or slave, Senator Lewis Cass and Senator Stephen A. Douglas promoted the idea of "popular sovereignty." That meant the people in the area would decide to have slavery or not (instead of politicians in Washington deciding). Popular sovereignty was a big part of the 1854 Kansas-Nebraska Act. That law undid the Missouri Compromise in practice. The Kansas-Nebraska Act also created the Kansas Territory and the Nebraska Territory.