What did the treaty establish?

The Treaty of Guadalupe Hidalgo of February 2, 1848 established the modern southern boundary of the United States, transferred approximately 525,000 square miles of Mexican territory to American sovereignty, set the framework for citizenship and property rights of Mexicans living in the ceded lands, and resolved territorial disputes that had triggered the Mexican-American War. The new boundary ran from the Gulf of Mexico up the Rio Grande, west across the Gila River and along the parallel north of the Gila to the Colorado River, down the Colorado, and west to the Pacific Ocean about one marine league south of San Diego Bay.

This boundary settled the dispute over Texas's southern border, which the United States had insisted was the Rio Grande and Mexico had argued was the Nueces River 100 miles to the north. By accepting the Rio Grande, Mexico relinquished its claim to about 100,000 square miles of present-day southern Texas. The treaty also transferred the entire Mexican Cession of about 525,000 square miles to American sovereignty: all of present-day California, Nevada, and Utah; most of Arizona and New Mexico; and parts of Colorado, Wyoming, and Kansas.

Combined with the Texas annexation of 1845 and the Oregon Treaty of June 15, 1846 that resolved the Pacific Northwest with Britain at the 49th parallel, the Treaty of Guadalupe Hidalgo gave the United States a continental geography stretching from the Atlantic to the Pacific. The remaining boundary issue, a small area along the southwestern New Mexico and Arizona border, was settled by the Gadsden Purchase of December 30, 1853, in which the United States paid 10 million dollars for an additional 30,000 square miles to facilitate construction of a southern transcontinental railroad route.

The treaty also established the legal framework for the approximately 80,000 Mexicans living in the ceded territories. Article VIII gave them one year to declare Mexican citizenship; those who did not were deemed to have elected United States citizenship. Article IX guaranteed them the rights of property and conscience, although in practice these guarantees were widely violated. Article X, which would have protected Mexican land grants more strongly, was struck by the United States Senate during ratification, leading to disputes that lasted decades. The Statement of Protocol of Queretaro on May 26, 1848 partially restored the deleted protections but lacked treaty force. The Court of Private Land Claims established by Congress in 1891 adjudicated remaining disputes through 1904.

Beyond formal text, the treaty established that the United States was now a transcontinental power, that the Pacific coast was American, and that the republic had grown through war and conquest as well as purchase and treaty.