Title: The Mexican-American War, 1846–1848 – U.S. History
Headings: 
The Mexican-American War, 1846–1848
Learning Objectives
JAMES K. POLK AND THE TRIUMPH OF EXPANSION
WAR WITH MEXICO, 1846–1848
CALIFORNIA AND THE GOLD RUSH
Section Summary
Glossary

Content: The Mexican-American War, 1846–1848 – U.S. History
A Nation on the Move: Westward Expansion, 1800–1860
The Mexican-American War, 1846–1848
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Learning Objectives
By the end of this section, you will be able to: Identify the causes of the Mexican-American War
Describe the outcomes of the war in 1848, especially the Mexican Cession
Describe the effect of the California Gold Rush on westward expansion
Tensions between the United States and Mexico rapidly deteriorated in the 1840s as American expansionists eagerly eyed Mexican land to the west, including the lush northern Mexican province of California. Indeed, in 1842, a U.S. naval fleet, incorrectly believing war had broken out, seized Monterey, California, a part of Mexico. Monterey was returned the next day, but the episode only added to the uneasiness with which Mexico viewed its northern neighbor. The forces of expansion, however, could not be contained, and American voters elected James Polk in 1844 because he promised to deliver more lands. President Polk fulfilled his promise by gaining Oregon and, most spectacularly, provoking a war with Mexico that ultimately fulfilled the wildest fantasies of expansionists. By 1848, the United States encompassed much of North America, a republic that stretched from the Atlantic to the Pacific. JAMES K. POLK AND THE TRIUMPH OF EXPANSION
A fervent belief in expansion gripped the United States in the 1840s. In 1845, a New York newspaper editor, John O’Sullivan, introduced the concept of “manifest destiny” to describe the very popular idea of the special role of the United States in overspreading the continent—the divine right and duty of white Americans to seize and settle the American West, thus spreading Protestant, democratic values.