Content Warning: The source material and this guide reference the enslavement of Black Americans and the associated racism and prejudice.

In the 1850s, the size of the United States had once again expanded to include lands won in the Mexican War (1846-48). This added to the lands acquired from France in the Louisiana Purchase (1803), which had already doubled the size of the country. A substantial gain had also been made in Oregon, which was ceded by Great Britain in 1846. Northern lands, because of their cooler, less arable character, were considered unfit for plantation agriculture. However, warmer, more fertile lands acquired from Mexico, but for the labor needs that could be filled by expanding slavery, held strong prospects for an expanded cash crop economy resembling that of the Southeast. Therefore, as the political geography of North America took shape, the issue of slavery loomed large, often framed as a necessity for the success of the regional economy. A map from the National Geographic Society provides a good visual aid to understand this crisis.

The Missouri Compromise (1820) forbade the expansion of slavery into any new states, save only Missouri, which was admitted as a slave state in exchange for the admission of Maine as a free state.