When the United States admitted Texas to the Union in 1845, the Mexican government was in such turmoil that the nation's new leader would not even meet with the Americans; they were too weak even to negotiate concessions. Both sides awaited the outbreak of violence. On May 9, 1846, President James K. Polk received word that Mexican forces had ambushed two of General Zachary Taylor's companies along the Rio Grande. He immediately demanded that Congress appropriate funds for war, proclaiming that the Mexicans had initiated a full- blown conflict. Somewhat reluctantly, Congress agreed, and the Mexican War began.
The Mexican War lasted one and a half years, and ranged all throughout Texas, New Mexico, and California, and even into the Mexican interior. Mexican resistance was stubborn and benefited from greater manpower than US forces, but ultimately proved futile. The US won an easy victory due to superior artillery and leadership. The Treaty of Guadalupe Hidalgo, signed February 2, 1848, ceded Texas, New Mexico, and California to the US, completing American claims to land all the way across the continent. In return, the US assumed all monetary claims of US citizens against the Mexican government and paid Mexico $15 million. The West was now officially open and secure to Americans.
Despite patriotism engendered by the war, sectional conflict grew more dramatic between 1846 and 1848. Not all of this was due to expansion. Polk created many enemies in the North through his lack of support for tariffs and in the West for his failure to initiate internal improvement. However, expansion and the future of slavery generated far greater conflict during the pre-Civil War era. Proslavery Democrats and antislavery Whigs raged against one another in Congress and in the press over the future of slavery in the expanded West.
Every solution to the problem of slavery created controversy. A Democratic congressman from Pennsylvania named David Wilmot introduced an amendment to an appropriations bill regarding the West known as the Wilmot Proviso. The proviso stated that slavery should be outlawed in all territory other than Texas ceded to the US by Mexico. Supported in the North, the proviso passed the House of Representatives but stalled in the Senate. Southern Democrats responded violently to any suggestion that slavery be abridged south of the line set by the Missouri Compromise: 36 degrees, 30 minutes latitude.
In the election of 1848, Zachary Taylor won the presidency as the Whig candidate. Both the Whigs and the Democrats tried to skirt the issue of slavery, the Whigs presenting no clear platform, and the Democrats endorsing the concept of popular sovereignty under which settlers would decide the issue of slavery for themselves. Soon the expansion Westward grew at such a rapid pace that politicians could no longer afford not to come up with a distinct decision regarding slavery. In January 1848, an American carpenter living at the base of the Sierra Nevada Mountains discovered gold in California. Within months, a frantic gold rush was in full swing. Overland immigrants to California totaled 400 in 1848, 25,000in 1849, and 44,000 in 1850.
Polk saw in a Mexican War the opportunity to advance toward California and New Mexico and complete the American sweep West. Reports from California suggested that the citizens there would accept American rule. Many Whig members of Congress believed that Polk was escalating a small skirmish into a call for general war for the purpose of expansion and the extension of slavery into the West. However, remembering that the Federalists had destroyed their party by opposing the War of 1812, many reluctantly went along with Polk's demands for appropriations.
By the end of the Mexican War, the spirit of expansion was especially strong. Some in Congress decried the Treaty of Guadalupe Hidalgo because it did not cede all of Mexico to the US after the resounding US victory. Others, however, argued that the racial impurity of Mexican inhabitants would lead to calamity. Thus, racism allowed Mexico to maintain its sovereignty.
As for the question of slavery in the West, which became the singular focus of US politics after the Mexican War, Polk believed that expansion would preserve the agricultural and democratic nature of the US, and weaken tendencies toward centralized power. He believed these benefits to be the paramount goal of westward expansion, and believed they would be reaped whether the new territory was free or slave. He saw the Missouri Compromise, which prohibited slavery in all land north of 36 degrees 30 minutes latitude, as a sufficient solution to the issue of slavery. Some antislavery Whigs vehemently disagreed, especially abolitionists from New England and Ohio, opposed the extension of slavery into the territories on moral grounds. However, a more important challenge to the expansion of slavery came from northern Democrats who feared that extending slavery into New Mexico and California would deter free laborers from settling there. They argued that deterring migration to the West would intensify class struggle in the East. David Wilmot fell into this second category. He was not an abolitionist, nor did he seek to split his party. He simply spoke for the northern Democrats who had been led to believe that Texas would be the last slave state. Polk and his cabinet had given the impression that Texas would be for the slaveholders and California and New Mexico for free labor.
The issue of slavery in the territories raised some questions of Constitutionality. John Calhoun and his followers asserted that since slaves were property, they should be protected in all areas by the Constitution, meaning that the Missouri Compromise was unconstitutional and slaveholders could take their slaves anywhere they wished. Northerners, on the other side, cited the history of regulation of slavery by the federal government and the wording of the Constitution, which gave Congress the power to "make all needful rules and regulations respecting the territory or other property of the United States." Politicians searched for a middle ground but more often than not found only morass and deadlock. The increasing expansion into the territories of the West, largely due to the gold rush, made the search for compromise all the more frantic.