Which act was designed to maintain a balance of power between slave and free states in the early 19th century?

The Missouri Compromise was a significant piece of legislation passed in 1820 aimed at maintaining a balance of power between slave and free states as the United States expanded westward. It allowed Missouri to enter the Union as a slave state while simultaneously admitting Maine as a free state, thereby preserving the balance of power in the Senate. Additionally, the compromise established a geographical boundary at latitude 36°30′ where slavery would be prohibited in the northern part of the Louisiana Territory. This helped to temporarily ease tensions between the North and South over the issue of slavery, highlighting the importance of legislative compromises in managing the growing sectional divides prior to the Civil War.

In contrast, the Kansas-Nebraska Act, passed in 1854, introduced the controversial idea of popular sovereignty, allowing settlers in those territories to determine whether they would allow slavery, which ultimately led to violent conflicts known as "Bleeding Kansas." The Compromise of 1850 sought to address similar issues but involved a series of laws rather than a single framework like the Missouri Compromise, and the Expatriation Act dealt with the issue of citizenship rather than balancing slave and free states.