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The process of admitting Texas as a slave state was well under way by the time Polk became president on March 4, 1845. One plank of the Democratic platform was thus resolved. In his first annual message to Congress, Polk asserted that the American claim to the entire Oregon country was “clear and unquestionable.” The British, who had refused on several occasions to relinquish any territory north of the Columbia River, now had a change of heart. Their chief fur-trading post had been moved to Vancouver Island, and British Minister Pakenham suggested extending the boundary line from the Rocky Mountains to the Pacific along the forty-ninth parallel. Polk, focusing on settling the Texas controversy and acquiring California, agreed to submit the British proposal to the Senate. On June 18, 1846, over the protests of expansionist Democratic senators demanding all of Oregon to the southern border of Alaska—“Fifty-four forty or fight”—the Oregon boundary settlement was ratified. Polk was especially pleased with the timing of the compromise, because the United States was already at war with Mexico.
Mexico broke diplomatic relations with Washington following the annexation of Texas, and continued to claim the Nueces River as the southwestern border of its rebellious province. Exacerbating the situation were millions of dollars in inflated claims that Americans had lodged against the Mexican government, and the driving desire of President Polk to acquire the valuable Pacific ports of California. Polk appointed John Slidell of Louisiana as minister to Mexico, and instructed him to offer up to 30 million dollars to settle the disputed claims and purchase California and New Mexico—the territory between Texas and California. Secretary of War William Marcy suggested to Thomas Larkin, the American consul in Monterey, that the Californios might follow the Texas example and declare their independence from Mexico. John Charles Frémont led an ostensible “exploring expedition” to support such a revolt.
The Polk administration failed in its initial efforts to acquire California and settle the Texas controversy. Californians did not rise in revolt, and Mexico rejected Slidell as an American minister. Polk then ordered General Taylor to move his troops across the Nueces to the Rio Grande, but the stalemate continued. On Saturday, May 9, 1846, the president informed his cabinet that the U.S. “had ample cause of war,” based upon the rejection of Slidell as minister and the claims issue. Only Secretary of the Navy George Bancroft, the preeminent historian of the age, opposed seeking an immediate declaration of war from Congress. That very evening, however, word was received that fighting had commenced along the Rio Grande. The following Monday, Polk declared that Mexico “invaded our territory, and shed American blood upon American soil.” Congress responded with a war resolution and an authorization for 50,000 volunteers.
The war with Mexico was popular in the Mississippi Valley, but was derided as “Mr. Polk’s War” in the northeast. Whigs generally opposed the war, but party members in Congress voted to support the American soldiers and marines during the fighting. Abraham Lincoln, a Whig congressman from Illinois, believed Polk rushed the country into war over the disputed territory between the Nueces and the Rio Grande. He demanded to know the exact “spot” the war started, but his views were not popular back home and he chose not to run for reelection.
Antislavery men naturally viewed the conflict as a brazen conspiracy to extend the boundaries of the "peculiar institution." James Russell Lowell, an abolitionist poet, castigated the Mexican War in the Biglow Papers:
Henry Davis Thoreau symbolically protested the war by refusing to pay his Massachusetts poll tax. He spent one night in the Concord jail, before his aunt paid his fine and he returned to Walden Pond to write a classic essay, “Civil Disobedience.” Thoreau rhetorically inquired: “How does it become a man to behave toward this American government today? I answer, that he cannot without disgrace be associated with it.”
Despite the opposition of Whigs and antislavery men, the war with Mexico was an unparalleled military success. After the first clash in late April, General Taylor crossed the Rio Grande and defeated numerically superior Mexican forces at the Battles of Palo Alto and Resaca de la Palma. Advancing on Monterrey, a town in northern Mexico, "Old Rough and Ready" and his men faced fierce house-to-house fighting against a valiant Mexican army led by General Pedro de Ampudia. Taylor agreed to a negotiated surrender, allowing the Mexican troops to retreat with their arms. President Polk countermanded the armistice, and ordered Taylor to take a defensive position and detach most of his veteran troops to bolster a planned attack against Mexico City. General Santa Anna tried to exploit Taylor’s weakened position, but the Battle of Buena Vista in February 1847 was a stunning American victory. It was also Taylor’s last fight—he returned home a military hero destined for the White House.
Polk’s main objective—California—was not the scene of major military action. Americans living near Sonoma raised the “Bear Flag Revolt” in June 1846, aided by Frémont’s small force. After his sailors and marines seized Monterey, Commodore John D. Sloat proclaimed the annexation of California and instituted a military government. Some Mexican loyalists resisted the American occupation, and sporadic fighting continued. Meanwhile, Colonel Stephen Kearney's small army garrisoned Santa Fe, New Mexico, before resuming their march. En route, Kearney encountered Kit Carson, who incorrectly reported that California had been pacified. Sending all but one hundred men back east, Kearney joined forces at San Diego with Commodore Robert Stockton and helped put down the loyalist revolt. The American forces entered Los Angeles in January 1847, ending the fighting in California.
The decisive campaign of the war was the expedition against Mexico City. Winfield Scott, the commanding general of the United States Army, landed his men on the beaches near Vera Cruz, and commenced a march that traced the route taken 300 years before by Cortés. Scott brushed aside Santa Anna’s army at Cerro Gordo, a battle in which Captains Robert E. Lee and George B. McClellan distinguished themselves. Santa Anna hastily recruited a Mexican army of about 20,000 troops, but many of them were ill-trained and equipped. In a series of sharp battles near the capital city, General Scott's army of nearly 14,000 men overwhelmed the Mexican forces. The fortified hill of Chapultepec was stormed despite the desperate resistance of the defenders, who included young military cadets known as “los niños." Mexico City fell on September 14, as American soldiers and marines entered the “halls of the Montezuma.”
Nicholas P. Trist, the chief clerk of the State Department, was sent by Polk to negotiate a peace treaty with the Mexican government. It was signed on February 2, 1848, at Guadalupe-Hidalgo. Mexico acknowledged the annexation of Texas (with the Rio Grande as its border), and ceded New Mexico and California to the United States. In return, the United States paid $15,000,000 for the Mexican Cession, and assumed up to $3,250,000 of the disputed claims. The war’s human toll included about 13,000 American dead—the vast majority due to diseases. In terms of the percentage of combatants, this remains the nation's costliest military conflict. It also reopened the slavery expansion controversy settled by the Missouri Compromise of 1820.
Ralph Waldo Emerson prophetically warned, “The United States will conquer Mexico, but it will be as the man swallows the arsenic, which brings him down in turn. Mexico will poison us.” Indeed, the Mexican Cession became a political battleground between the North and the South. The issue was raised early in the war by David Wilmot, a Democratic congressman from Pennsylvania. Employing the language of the Northwest Ordinance, Wilmot proposed that slavery be prohibited in any territory acquired from Mexico. The “Wilmot Proviso” passed the House frequently in the next several years, but it was always defeated in the Senate. It never became law, but represented the extreme Northern position regarding the extension of slavery.
Senator John C. Calhoun presented the extreme Southern position on slavery expansion in February 1847. Calhoun argued that Congress had no power to prohibit slavery in any American territory, and Southerners subsequently demanded that federal slave codes protect slavery in the Mexican Cession. Two compromise proposals were also advanced prior to the election of 1848. James Buchanan urged that the Missouri Compromise line of 36o 30’ be extended to the Pacific. President Polk agreed; but it was becoming more difficult for politicians to concede any territory in the fight over slavery. The other compromise proposal, known as “popular sovereignty,” was introduced in December 1847 by Lewis Cass, a moderate Democratic senator from Michigan. Cass adroitly proposed that the explosive slavery question be removed from the halls of Congress by letting the people of the territories decide the matter. As it turned out, a decision would have to be reached soon because of the California gold rush.
Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education