1. The Mexican-American War: The war, fought from 1846 to 1848, resulted in the United States acquiring a vast amount of territory, including present-day Texas, New Mexico, Arizona, California, and parts of Colorado, Utah, and Nevada. This raised the question of whether slavery would be allowed in these newly acquired territories, and whether new states carved out of these lands would be free states or slave states.
2. The Wilmot Proviso: In 1846, Congressman David Wilmot of Pennsylvania proposed an amendment to a bill that would have appropriated money for the war with Mexico. The Wilmot Proviso stipulated that slavery would be prohibited in any territory acquired as a result of the war. Although the Wilmot Proviso failed to pass, it became a rallying cry for those opposed to the expansion of slavery.