The Wilmot Proviso was a legislative proposal to ban slavery in any territory gained by the United States as a result of the Mexican-American War. Though it did not pass, it inflamed the existing sectional differences between the slave-holding southerners and abolitionist northerners.
Named after Pennsylvania Congressman David Wilmot, the Wilmot Proviso reflected fears among northerners that the Mexican-American War would vastly increase the number of slave states in the United States and decrease the influence of the abolitionist states. Wilmot made his proposal in 1846, 1847 and 1848, but it never passed. However, the idea of limiting the number of new slave states inspired people across the nation, and it launched the formation of the Republican Party in 1854.