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.
