The Mexican-American War ranks among the most consequential events in the history of both nations. Although the casus belli for the United States’s May 12, 1846, declaration of war was the Mexican ambush of a U.S. Army patrol in the disputed Nueces Strip on April 25 of that year, two underlying causes rendered conflict inevitable. The dispute over Texas was the first, and the desire of both nations to control the Mexican provinces of Nuevo Mexico and California was the second. 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 events on the battlefields. In México, a devastating war of independence (1810–1821), multiple violent overthrows of the federal government, the failure of two constitutions to produce a structure acceptable to both conservatives and liberals, and enmities generated by the socioeconomic structure severely limited México’s growth, tranquility, and potential for armed resistance to an invader. 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 deployed in later phases of the war fought in Texas and in the Mexican provinces of Nuevo Mexico, California, Tamaulipas, and Nuevo Leon. When United States 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 in Mexico City on September 14, 1847.
In the following seven months, 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 525,000 square miles of territory, the 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 reestablishment of Mexican sovereignty over territories in rebellion.