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United States Expansion, War with Mexico

Bear Flag Republic, Mexican Cession, General Zachary Taylor, Treaty of Guadalupe Hidalgo, expansionists

There was ample reason for that suspicion. While Polk compromised with Britain on the Oregon boundary, he stood adamant against Mexico on the question of Texas. Mexico warned that it would consider the annexation of Texas by the United States a declaration of war. A Texas convention voted to join the Union on July 4, 1845. Polk and a Congress strongly favoring annexation not only offered to take Texas into the Union, they also set the southern boundary of the new state at the Rio Grande—150 miles south of what most people had agreed was the Texas–Mexico border. The new boundary gave Texas far more Mexican land (including much of present-day New Mexico and Colorado) than the Texas Revolution had given it. Polk knew that the additional territory would provide a gateway to New Mexico and California, territories of northern Mexico that he and other expansionists coveted along with Texas. While annexing Texas, Polk offered to buy New Mexico and California from Mexico for $30 million in late 1845—an offer that the Mexicans angrily refused. Polk then provoked a war with Mexico in which he would win all that he had offered to buy.

As Mexico prepared for war, Polk sent troops into the disputed area north of the Rio Grande. Mexico sent troops north of the Rio Grande and in spring 1846 fought a skirmish in which the Americans suffered more than a dozen casualties. Congress declared war on Mexico that May. Near–unanimous congressional support for the declaration hid the fact that most Whigs and many Northern Democrats were deeply suspicious of a Southern war to annex new territory for slavery.

In the war the Americans launched a three–pronged offensive. General Zachary Taylor invaded northern Mexico from Texas, capturing the city of Monterrey in September 1846. A second American army under General Stephen Kearny occupied Santa Fe in August of that year. Kearny then sent part of his force to join Taylor at Monterrey and marched the rest of his army west to California, where American settlers had already established an independent “Bear Flag Republic.” At the same time, the U.S. Navy seized California ports.

Having lost Texas, California, New Mexico, and large portions of Chihuahua and Sonora in northern Mexico, the Mexicans marched toward Taylor’s army near Monterrey. Taylor held off determined attacks by a Mexican army about three times as large as his own and won the Battle of Buena Vista in February 1847. The next month the third prong of the U.S. offensive was launched when General Winfield Scott landed at Veracruz. Five months later he had fought his way to Mexico City.

As happened in much of the war, the Mexican army was larger and fought bravely, but the Mexican government and high command were divided and often incompetent, and the Americans were better armed and better led. In particular, the Mexicans had no answer to American artillery. After a series of bloody battles in September 1847, Scott’s army occupied Mexico City, and the war was over.

The Treaty of Guadalupe Hidalgo in 1848 ceded Texas (with the Rio Grande boundary), California, and New Mexico to the United States, which agreed to pay Mexico $15 million. The Mexican Cession gave the United States present–day west Texas, New Mexico, Arizona, California, Nevada, Utah, most of Colorado, and part of Wyoming. The northern third of Mexico had become the southwestern quarter of the United States.

The Mexican War was a straightforward land–grab. The ease with which the United States won and the arrogance with which it behaved created a distrustful and sometimes violent southern border area for the country. More immediately, the lands ceded by the Treaty of Guadalupe Hidalgo became the object of contest and resentment between the slave and free states—a conflict that would widen into the American Civil War 13 years later.

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