Nine years after winning independence from Mexico, the Republic of Texas was formally annexed by the United States, becoming the 28th state. Mexico had never recognized Texan independence and viewed annexation as an act of aggression. The dispute over the Texas-Mexico border soon escalated into the Mexican-American War, which would ultimately reshape the map of North America.
Texas had spent nearly a decade as an independent republic after breaking from Mexico in 1836, unable to secure annexation earlier largely because its admission as a slave state threatened the fragile North-South balance in Congress. By 1845, expansionist fever and the doctrine of Manifest Destiny had shifted political will, and President John Tyler pushed through a joint resolution just before leaving office; his successor James K. Polk signed the final proclamation admitting Texas as a state.
Mexico, which had never accepted Texan independence, severed diplomatic relations with Washington immediately. The two nations disagreed sharply over the state's southern border — Texas claimed the Rio Grande, Mexico insisted on the Nueces River further north, leaving a disputed strip of land. Polk sent troops into this zone, and within months fighting broke out, providing the pretext for the Mexican-American War of 1846-48. That war's outcome would transfer California, Nevada, Utah, and parts of several other future states to the US, making Texas's admission one of the single most consequential territorial decisions in American history.