by Drew VandeCreek, Ph.D.

After the conclusion of the Mexican-American war, the American diplomat Nicholas Trist negotiated the treaty of Guadalupe-Hidalgo, in which the United States claimed over 500,000 square miles of new territory. These lands included Texas, as well as the Mexican territories of New Mexico and Upper California. Eventually they would become the American states of California, Arizona, and New Mexico, and comprise significant parts of Utah, Colorado, Nevada, and Wyoming. For this territory Mexico received $15 million, as well as $3,250,000 to settle American citizens' claims against the Mexican government. The United States Senate ratified the treaty on March 10, 1848. The Mexican Congress approved it on May 25.

The new territories acquired in the Mexican-American War quickly stoked the flames of sectional controversy in American national politics. Even as the fighting continued, northerners partial to free labor and southerners seeking the expansion of slavery began to quarrel over the fate of the impending acquisitions. In August of 1846 the Pennsylvania Democratic Congressman David Wilmot introduced his Proviso, which sought to prohibit the introduction of slavery into any territory gained by the war. Northern Democrats supported the Wilmot Proviso because it allowed them to support the popular war without advancing the cause of slavery's expansion. Southerners reacted angrily to the Proviso, declaring that it represented a northern conspiracy against their interests. Although Congress defeated the Proviso in 1846, it reappeared again in bills to supply troops and conclude the war. Although it never became law, Wilmot's proposal quickly split the Democratic Party into northern and southern wings and paved the way to Civil War.