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date: 21 September 2017

The Mexico-United States War

This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Latin American History. Please check back later for the full article.

The Mexico-United States War ranks among the most consequential events in the history of both nations. Although the casus belli for the declaration of war by the United States, on May 12, 1846, was the Mexican ambush of a U.S. Army patrol in the disputed Nueces Strip on April 25 of that year, the underlying cause was the desire of both nations to control the Mexican provinces of Nuevo Mexico and California. President James Knox Polk identified the acquisition of that territory as the principal objective of his administration.

The conflict also remains noteworthy for the extent to which the political milieu in both countries proved as important as the events on the battlefields. Mexico’s unity and growth had been severely set back by a devastating war of independence (1810–1821), by multiple golpes del estado, by the failure of two constitutions to produce a structure acceptable to both Conservatives and Liberals, and by enmities generated by the socio-economic structure. In the United States, the national unity evident at the outbreak of the war faded in the face of sectional rivalries, unexpectedly high casualties, and declining relations between the executive and legislative branches.

The military phases of the war fall into two segments. In the first, forces considerably smaller than those that would later be employed fought in Texas and in the Mexican provinces of Nuevo Mexico, California, Tamaulipas, and Nuevo Leon. When U.S. victories in northern Mexico failed to produce the anticipated Mexican surrender, the second phase of the conflict began on March 9, 1847, with General Winfield Scott’s invasion of central Mexico, and ended with his entrance into Mexico City on September 14, 1847.

In the seven months following, both governments sought to obtain the best terms. A rising tide of violent rural rebellion in Mexico and a rising tide of Whig opposition to the Polk administration in Washington served as catalysts during the negotiations. Two agreements, the February 2, 1848 Treaty of Guadalupe-Hidalgo and the March 6, 1848 Truce Agreement brought hostilities a close.

Consequences of the conflict included the Mexico’s loss of 529,000 square miles of territory, the consequent emergence of the United States as the dominant continental power, the dispossession of many Mexican citizens living in what had become U.S. territory, and the re-establishment of Mexican sovereignty over territories in rebellion.