To achieve national unity, the country’s founders did not confront the issue of slavery in the Constitution, but left it for future generations to resolve. In the nineteenth century, applications for statehood from western territories threatened the balance of free and slave states in the Senate. When Missouri, which allowed slavery, applied for statehood in 1819, Congress struggled for a way to maintain the Union despite strongly opposing pro- and antislavery constituencies. After heated debate, Congress adopted the Missouri Compromise, which admitted Maine as a free state to balance Missouri and prohibited slavery north of the 36º 30’ latitude in the Louisiana Territory. Congress shifted the basis for determining the status of slavery in each territory from the territory’s geographical location to a popular vote by its inhabitants. Slavery remained a burning issue that threatened the Union and led to the Civil War.