The Mexican War (1846–48) was officially ended when the U.S. Senate ratified the Treaty of Guadalupe Hidalgo on March 10, 1848. By the treaty, Mexico relinquished roughly half its territory—New Mexico and California—to the United States. Mexico also recognized the Rio Grande as its border with Texas.

Mexico received payments in the millions from the United States, which also assumed the payment of claims of its citizens. Five years later under the terms of the Gadsden Purchase, the United States purchased a small portion of land from Mexico for another $10 million, which was widely regarded as further compensation for the land lost in the war. The territory the United States gained was in present-day Arizona and New Mexico, south of the Gila River.