The Mexican-American War, 1846–1848

Section Summary

President James K. Polk’s administration was a period of intensive expansion for the United States. After overseeing the final details regarding the annexation of Texas from Mexico, Polk negotiated a peaceful settlement with Great Britain regarding ownership of the Oregon Country, which brought the United States what are now the states of Washington and Oregon. The acquisition of additional lands from Mexico, a country many in the United States perceived as weak and inferior, was not so bloodless. The Mexican Cession added nearly half of Mexico’s territory to the United States, including New Mexico and California, and established the U.S.-Mexico border at the Rio Grande. The California Gold Rush rapidly expanded the population of the new territory, but also prompted concerns over immigration, especially from China.