Polk's Texas annexation war redrew the map to the Pacific
The Mexican–American War (25 April 1846 – 2 February 1848)—called the Mexican War in the U.S. and an intervention in Mexico—followed 1845 Texas annexation that Mexico still claimed. Slavery politics had blocked earlier annexation lest Texas tip the Senate balance; Democrat James K. Polk won 1844 on expansion.
Mexico entered the war fragile. Ten years of war against Spanish royal troops ended with the Treaty of Córdoba in 1821, but the struggle wrecked the silver districts of Zacatecas and Guanajuato, flirted briefly with monarchy and became a republic in 1824. Its sparsely settled, arid north was barely governed; once Spanish funding for presidios and payments to Native nations stopped, Comanche and Apache raiders ranged hundreds of miles south for livestock, and New Mexico came to depend on trade along the Santa Fe Trail.
Texas was the fuse. Mexico had invited American settlers into Tejas to push into Comanche country, but most chose the fertile east beside the slave states, and Mexico's 1829 abolition of slavery failed to slow the influx. After the 1836 revolution, Santa Anna signed the Treaties of Velasco as a captive, and Mexico never accepted them. The border was contested too: Texas and Washington said the Rio Grande, Mexico said otherwise.
Historians weigh the motives differently. Peter Guardino calls American prosperity, fuelled by demand for cotton picked by enslaved people, the decisive advantage, and he also points to racial and gendered stereotypes casting Mexicans as weaker. Rachel John stresses ideology: a young republic eager for prestige saw a neighbour unable to hold its own territory. Already in 1842 the American envoy Waddy Thompson Jr. had praised California as worth far more than Texas.
Militarily the outcome came fast. Winfield Scott landed at Veracruz on 9 March 1847 and took Mexico City that September, though some Mexican factions resisted making peace. West Point graduates gained experience they would later use as Civil War commanders, and in Mexico the defeat deepened political turmoil and cost the nation prestige.
Source: Mexican–American War
