More than 60,000 volunteers enlisted and did most of the fighting. Combat took place on three fronts. In June 1846, a band of American insurrectionists proclaimed California freed from Mexican control and named Captain John C. Fremont, head of a small scientific expedition in the West, its ruler. Their aim was California’s incorporation into the United States, but for the moment they adopted a flag depicting a large bear as the symbol of the area’s independence. A month later, the U.S. Navy sailed into Monterey and San Francisco Harbors, raised the American flag, and put an end to the “bear flag republic.” At almost the same time, 1,600 American troops under General Stephen W. Kearney occupied Sante Fe without resistance and then set out for southern California, where they helped to put down a Mexican uprising against American rule.

The bulk of the fighting occurred in central Mexico. In February 1847, Taylor defeated Santa Anna’s army at the Battle of Buena Vista. When the Mexican government still refused to negotiate, Polk ordered American forces under Winfield Scott to march inland from the port of Vera Cruz toward Mexico City. Scott’s forces routed Mexican defenders and in September occupied the country’s capital. In February 1848, the two governments agreed to the Treaty of Guadalupe Hidalgo, which confirmed the annexation of Texas and ceded California and present-day New Mexico,

The Mexican War was the first in which an American army invaded another country and occupied its capital As a result of the war, the United States acquired a vast new area in the modern-day Southwest.

Arizona, Nevada, and Utah to the United States. In exchange, the United States paid Mexico $15 million. The Mexican Cession, as the land annexed from Mexico was called, established the present territorial boundaries on the North American continent except for the Gadsden Purchase, a parcel of additional land bought from Mexico in 1853, and Alaska, acquired from Russia in 1867.

The Mexican War is only a footnote in most Americans’ historical memory. Unlike other wars, few public monuments celebrate the conflict. Mexicans, however, regard the war (or “the dismemberment,” as it is called in that country) as a central event of their national history and a source of continued resentment over a century and a half after it was fought. As the Mexican negotiators of 1848 complained, it was unprecedented to launch a war because a country refused to sell part of its territory to a neighbor.

A map of the United States from 1848 reveals how the size of the country had grown during the past four years: Texas (its western boundary still unfixed) had been annexed in 1845; the dispute with Great Britain over Oregon was settled in 1846; and the Mexican Cession—the area of present-day Arizona, New Mexico, Utah, Nevada, and California—was added in 1848 at the end of the Mexican War.

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