Answer: Acquired after the Missouri Compromise, which did not include those territories.
Explanation:
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 Missouri Compromise (1820) had admitted Missouri into the Union as a slave state with Maine being added at the same time to keep the balance of slave and free states equal. It also prohibited any future slave states north of the 36/30' latitude line 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. Since that latitude line ran right through the middle of the Mexican Cession territory, and because the Missouri Compromise had only addressed lands acquired in the Louisiana Purchase territories, there was bound to be further debate over the issue of slave vs. free states.
