The growing sectional division in the country over the issue of slavery.
The Missouri Compromise (1820) admitted Missouri into the Union as a slave state with Maine being added at the same time as a free state, to keep the balance of slave and free states equal. The Missouri Compromise also prohibited any future slave states north of the latitude line 36 1/2 degrees north of the equator in territories of the Louisiana Purchase, with the exception of Missouri (north of that line) being admitted as a slave state.
A couple decades later, that sectional debate was sparked still further by the acquisition of lands from Mexico after the Mexican-American War. The Mexican Cession was the large region of land that Mexico ceded to the United States in the Treaty of Guadalupe Hidalgo in 1848. It included territory that would later become the states of California, Nevada, Utah, and parts of what would become Arizona, New Mexico, Colorado and Wyoming. The Mexican Cession reignited tension on the issue of slave-holding states vs. free states. Since the Missouri Compromise had specified only the Louisiana Purchase lands with its 36 1/2 degrees latitude dividing line, new debate arose over whether territories in the Mexican Cession territory would be slave or free states.
