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Map of the United States including Western Territories, December, 1848

This map, which accompanied President James K. Polk’s last annual message to Congress in December 1848, shows the more than half-million square miles the United States gained from the Mexican War. The Wilmot Proviso would have prohibited slavery in the territories of California, New Mexico, and Texas—an area that now encompasses all or parts of eight states.

Records of the U.S. House of Representatives, National Archives and Records Administration

Map of the United States including Western Territories, December, 1848

Slavery in the West

Westward expansion stoked tensions in Congress as Northern and Southern factions disputed whether to allow slavery in new territories. In 1846, during the Mexican War, Representative David Wilmot of Pennsylvania attempted to limit slavery in the West through an amendment to a war appropriations bill. His proviso, banning slavery in any territories acquired from Mexico, passed the House but not the Senate. The Wilmot Proviso––included in subsequent House bills but never enacted—kept slavery at the forefront of congressional debate.