conflict between the U.S. and Mexico, lasting from 1846 to 1848. The principal causes of the war were the annexation of Texas (Dec. 29, 1845) by the U.S.; claims against the Mexican government by U.S. citizens who had been injured and whose property had been damaged during the frequent Mexican revolutions of the period; and the desire of the U.S. to acquire California, a Mexican province in which many U.S. citizens had settled and which the U.S. feared might fall under British or French rule. In November 1845, President James K. Polk sent the diplomat John Slidell to Mexico to seek boundary adjustments in return for the U.S. government’s settlement of the claims of U.S. citizens against Mexico, and also to make an offer to purchase California and New Mexico. The Mexican authorities refused to negotiate with Slidell. After the failure of this mission, a U.S. army under Gen. Zachary Taylor advanced to the mouth of the Rio Grande, the river that the state of Texas claimed as its southern boundary. Mexico, claiming that the boundary was the Nueces River, to the northeast of the Rio Grande, considered the advance of Taylor’s army an act of aggression and in April 1846 sent troops across the Rio Grande. Polk, in turn, declared the Mexican advance to be an invasion of U.S. soil, and Congress declared war on Mexico on May 13, 1846.

The U.S. plan of campaign was threefold, involving an invasion of northern Mexico by Taylor, occupation of New Mexico and California by forces under Col. Stephen W. Kearny, and a blockade of both Mexican coasts. Even before war had been formally declared Taylor won the battles of Palo Alto (May 8, 1846) and Resaca de la Palma (May 9) and forced the Mexicans back across the Rio Grande. He then advanced into Mexico. Taylor occupied Matamoros in present-day Tamaulipas (May 18), captured Monterrey (September 24), and by defeating the Mexican forces under Gen. Antonio Lopez de Santa Anna in a stubbornly contested battle at Buena Vista (Feb. 22–23, 1847), put an end to Mexican resistance in northern Mexico. Kearny occupied what is now New Mexico and then, advancing into California, helped in the occupation of that territory. Under the leadership of the U.S. naval officer Commodore John Drake Sloat (1781–1867) and Capt. John C. Frémont of the U.S. Army, California had already declared its independence from Mexico and was declared a territory of the U.S. in July 1846.

In spite of these U.S. victories and the success of the blockade, Mexico refused to acknowledge defeat, and the U.S. decided to send an expedition to end the war by capturing Mexico City. American troops under Gen. Winfield Scott took Vera Cruz (March 29, 1847) and defeated the Mexicans at Cerro Gordo, Contreras, and Churubusco. U.S. troops next took Casa Mata and Molino del Rey and then stormed the hill of Chapultepec, the key to Mexico City, which fell on Sept. 14, 1847. Peace was established by the Treaty of Guadalupe Hidalgo, signed on Feb. 2, 1848. The Rio Grande was made the southern boundary of Texas, and California and New Mexico were ceded to the U.S. In return the U.S. paid Mexico the sum of $15 million and agreed to settle all claims of U.S. citizens against Mexico.

For further information on this topic, see the Bibliography, section 1141. The Nineteenth Century.