To Authorize the Extension of Slavery Where It Has Previously Been Totally Abolished: The Significance of Abolition in Mexico to Sectional Controversy in the United States, 1846–50

If any event in American history was responsible for the sectional conflict that led to the Civil War, it was the introduction of the Wilmot Proviso. Yet historians struggle to explain why a Democrat from Pennsylvania who never gave any sign of antislavery convictions made such a proposal and why so many Northern Democrats broke with their party to support it. This paper argues that Mexico’s 1837 abolition of slavery is key to understanding the Proviso. Although most of the Mexican cession lay below 36^o30’, even northerners who had supported the annexation of Texas without any restrictions on slavery would not reestablish the Peculiar Institution where it had been abolished. This argument armed Northerners with the same oratorical ammunition that Southerners had directed against them—to “let slavery alone” and take territories “as they came.” Recognizing the importance of Mexico’s abolition of slavery to the Wilmot Proviso is important for two reasons. First, the constraints that Mexican statutes placed on the status of slavery in the ceded territories demonstrates that laws could have profound consequences, regardless of how evenly they were enforced. Second, the fact that Mexican ideas about freedom helped to destabilize the fragile balance between North and South offers a corrective to the common account of American expansion in the mid-nineteenth century--an account in which the United States government simply imposed its will upon its Latin American neighbors.