The Mexican War of 1846-1848 broke out ostensibly over a dispute about the Texan boundary. But it really originated in the expansionist spirit of “manifest destiny,” of which President James K. Polk was a leading proponent. Polk came to office determined to acquire the Mexican provinces of California and New Mexico and when Mexico refused to sell them, used a border clash as a pretext for war. Although American opinion was deeply divided over the conflict, the war turned out to be a succession of triumphs for American arms. The two provinces Polk coveted were easily overrun: General Zachary Taylor overwhelmed a much larger Mexican army at Buena Vista (February 1847); and General Winfield Scott’s seaborne expedition to Vera Cruz fought its way into the heart of Mexico against superior forces and captured Mexico City (September 1847), forcing Mexico to make peace. By the Treaty of Guadalupe-Hidalgo (February 1848), Mexico ceded the territories of California and New Mexico and acknowledged the Rio Grande as the Texas boundary. While the war brought the U.S. vast territorial gains — extending America’s western boundary to the Pacific — it also revived the simmering conflict over slavery and its extension to new territories.

Although proclaimed part of New Spain as early as 1542, California became a field for Spanish colonization only in the late 18th century when Franciscan friars, led by Father Junipero Serra, established a series of missions on the coast between San Diego and San Francisco. Father Serra’s priests attempted to convert the Indians to Christianity and teach them to become farmers and herders. In the process however, they imposed a new, alien and strictly regulated life-style, which undermined tribal culture. After Mexico became independent from Spain in 1821, the Mexican government began secularizing the missions and between 1833 and 1840, it parceled out the mission ranches to political favorites. The wealthy rancheros raised cattle, tended by vaqueros (cowboys), and sold hides and tallow to merchant ships from New England — a trade vividly described in Richard Henry Dana’s 1846 classic, Two Years Before the Mast. Lured by favorable publicity about the area, including John C. Fremont’s glowing reports of his expeditions, Americans began settling in California in the 1840s. Although numbering only about 700, they were soon toying with thoughts of independence and California’s annexation by the U.S. — ambitions promptly realized when rumors of the outbreak of the Mexican War in 1846 prompted the Bear Flag Revolt.

Born: 1795, Mecklenburg County, NC
Died: 1849

James Polk was a Tennessee lawyer, Governor and U.S. Speaker of the House before he emerged as the Democrats’ compromise Presidential candidate in 1845, defeating the Whig leader Henry Clay. The “dark horse” President claimed America had a “manifest destiny” to expand across the continent. The public agreed, but his own party was divided on the issue, knowing that Polk’s vision could well result in war with Mexico or Britain.

The Mexican War began in 1846 after Texas joined the Union. Triumphant American forces entered Mexico City in 1847. The Guadalupe-Hidalgo Treaty of 1848 established the Texas border at the Rio Grande and provided for America’s purchase of a huge tract of land encompassing what is now California and Nevada and parts of Arizona, New Mexico and Utah. War with Britain in the Northwest was averted by the Oregon Treaty of 1846, which fixed most of that territory’s boundary at the 49th parallel.

Polk fulfilled all his campaign pledges, including lowering the national tariff and establishing an independent Treasury. The Presidency took a toll on his health, however, and he died three months after leaving office.

Eleventh President
Democrat