After the defeat of its army and the fall of the capital in September 1847, Mexico entered into peace negotiations with the U.S. envoy, Nicholas Trist.

Mexicans in areas annexed by the U.S. could relocate within Mexico's new boundaries or receive American citizenship and full civil rights.

Trist negotiated with a special commission representing the collapsed government led by José Bernardo Couto, Miguel de Atristain, and Luis Gonzaga Cuevas of Mexico.

Unlike the New Mexico segment of the boundary, which depended partly on unknown geography, "to preclude all difficulty in tracing upon the ground the limit separating Upper from Lower California", a straight line was drawn from the mouth of the Gila to one marine league south of the southernmost point of the Port of San Diego, slightly north of the previous Mexican provincial boundary at Playas de Rosarito.

The Mexican Cession included essentially the entirety of the former Mexican territory of Alta California, but only the western portion of Santa Fe de Nuevo Mexico, and includes all of present-day California, Nevada and Utah, most of Arizona, western portions of New Mexico and Colorado, and the southwestern corner of Wyoming.

Articles VIII and IX ensured the safety of existing property rights of Mexican citizens living in the transferred territories.

Despite assurances to the contrary, land grants by the Mexican government to its citizens were often not honored by the United States because of unilateral modifications to and interpretations of the Treaty and U.S. legal decisions.

Mexicans believed that the United States had encouraged and assisted the Comanche and Apache raids that had devastated northern Mexico in the years before the war.

About 80,000 Mexicans inhabited California, New Mexico, Arizona, and Texas during the period 1845 to 1850, with far fewer in Nevada, southern and western Colorado, and Utah.

[19] On 1 March 1845, U.S. President John Tyler signed legislation to authorize the United States to annex the Republic of Texas, effective on 29 December 1845.

The Mexican government, which had never recognized the Republic of Texas as an independent country, had warned that annexation would be viewed as an act of war.

Both the United Kingdom and France recognized the Republic of Texas's independence and repeatedly tried to dissuade Mexico from declaring war against its northern neighbor.

President Polk's State of the Union address in December 1847 upheld Mexican independence and argued at length that occupation and any further military operations in Mexico were aimed at securing a treaty ceding California and New Mexico up to approximately the 32nd parallel north and possibly Baja California and transit rights across the Isthmus of Tehuantepec.

Even with its capital under enemy occupation, the Mexican government was inclined to consider factors such as the unwillingness of the U.S. administration to annex Mexico outright and what appeared to be deep divisions in domestic U.S. opinion regarding the war and its aims, which caused it to imagine that it was actually in a far better negotiating position than the military situation might have suggested.

Anglo-American settlers already dominated this territory, but perhaps more importantly from the Mexican point of view, it represented the bulk of pre-war Mexican territory north of the Missouri Compromise line of parallel 36°30′ north — lands that, if annexed by the United States, would have been presumed by Northerners to be forever free of slavery.

[citation needed] While the Mexican government could not reasonably have expected the Polk Administration to accept such terms, it would have had reason to hope that a rejection of peace terms so favorable to Northern interests might have the potential to provoke sectional conflict in the United States or perhaps even a civil war that would fatally undermine the U.S. military position in Mexico.

[28] Notwithstanding that the treaty had been negotiated against his instructions, given its achievement of the major American aim, President Polk passed it on to the Senate.

[28] The Treaty of Guadalupe Hidalgo was signed by Nicholas Trist (on behalf of the United States) and Luis G. Cuevas, Bernardo Couto, and Miguel Atristain as plenipotentiary representatives of Mexico on 2 February 1848 at the main altar of the old Basilica of Guadalupe at Villa Hidalgo (within the present city limits) as U.S. troops under the command of Gen. Winfield Scott were occupying Mexico City.

Article VIII guaranteed that Mexicans who remained more than one year in the ceded lands would automatically become full-fledged United States citizens (or they could declare their intention of remaining Mexican citizens); however, the Senate modified Article IX, changing the first paragraph and excluding the last two.

An amendment by Jefferson Davis giving the United States most of Tamaulipas and Nuevo León, all of Coahuila, and a large part of Chihuahua was supported by both senators from Texas (Sam Houston and Thomas Jefferson Rusk), Daniel S. Dickinson of New York, Stephen A. Douglas of Illinois, Edward A. Hannegan of Indiana, and one each from Alabama, Florida, Mississippi, Ohio, Missouri, and Tennessee.

Most of the leaders of the Democratic party, Thomas Hart Benton, John C. Calhoun, Herschel V. Johnson, Lewis Cass, James Murray Mason of Virginia and Ambrose Hundley Sevier were opposed, and the amendment was defeated 44–11.

A motion to insert into the treaty the Wilmot Proviso (banning slavery from the acquired territories) failed 15–38 on sectional lines.

He was detained in a Senate committee room for one month, though he continued to file articles for his newspaper and ate and slept at the home of the sergeant at arms.

Meanwhile, most notably in the Federal District there was a Mexican element advocating annexation of the entire country to the United States.

On 26 May 1848 the government received the commissioners Nathan Clifford and Ambrose Hundley Sevier who were in Mexico to negotiate the treaty after congress had approved it with some slight modifications.

[36] On 30 May 1848, when the two countries exchanged ratifications of the treaty of Guadalupe Hidalgo, they further negotiated a three-article protocol to explain the amendments.

[39] In addition to the sale of land, the treaty also provided recognition of the Rio Grande as the boundary between the state of Texas and Mexico.

The southern border of California was designated as a line from the junction of the Colorado and Gila rivers westward to the Pacific Ocean so that it passes one Spanish league south of the southernmost portion of San Diego Bay.

[42] Community property rights in California and other western states are based on the Visigothic Code which Spain adopted and then brought to the Americas, including the former territories of Mexico that were ceded to the U.S.

Once viewed as a model of international cooperation, in recent decades, the IBWC has been heavily criticized as an institutional anachronism, bypassed by modern social, environmental, and political issues.