The annexation of Texas to the United States in 1845 and the conquest of what became the American Southwest in the Mexican-American War of 1846–1848 reduced the size of Mexico by more than half and increased the size of the United States by a third. These acquisitions also reopened the vexed question of slavery’s expansion, which supposedly had been settled by the Missouri Compromise of 1820.
Three months into the Mexican War, Pennsylvania Congressman David Wilmot introduced his famous “Proviso” stipulating that in any territory acquired from Mexico “neither slavery nor involuntary servitude shall ever exist in any part of said territory.” Almost unanimous support by Northern congressmen, Whig and Democrat alike, passed this resolution over the virtually unanimous opposition of representatives of both parties from the South. In the Senate, however, the equal representation of the fifteen slave states and fifteen free states enabled Southern senators to block Wilmot’s Proviso.
These events sounded an ominous knell for the future of the republic. Congressional votes normally divided along party lines, with Northern and Southern Democrats lining up together against Northern and Southern Whigs. The wrenching of this partisan pattern into a sectional split on the Wilmot Proviso foreshadowed the increasing polarization that finally plunged the nation into disunion and civil war fifteen years later.
It was all so unnecessary, according to historians whose interpretation of the Civil War’s causes once prevailed. With the expansion of the cotton frontier into eastern Texas in the 1830s, they maintained, slavery had reached the “natural limits” of its growth and could spread no farther into the arid and inhospitable Southwest.
The Natural Limits thesis echoes the voices of antebellum politicians exasperated by antislavery claims that slaveholders intended to expand their “peculiar institution” into the territory taken from Mexico. This whole matter, insisted one Southern congressman, “related to an imaginary negro in an impossible place.” Kentucky Governor John J. Crittenden maintained in 1848 that “the right to carry slaves to New Mexico or California …