Annexation and Border Disputes

Mexico denounced the annexation of Texas as “an act of aggression, the most unjust which can be found recorded in the annals of modern history.”^[1] Beyond the anger produced by the annexation, the two nations both laid claim over a narrow strip of land between the Nueces River, where Mexico drew the southwestern border of Texas, and the Rio Grande, roughly 150 miles farther west, where Texans claimed that the border lay. Neither claim was realistic since the sparsely populated area, known as the Nueces Strip, was in fact controlled by Native Americans.

Figure 1. In 1845, when Texas joined the United States, Mexico insisted the United States had a right only to the territory northeast of the Nueces River. The United States argued in turn that it should have title to all land between the Nueces and the Rio Grande.

In November 1845, President Polk secretly dispatched John Slidell to Mexico City to purchase the Nueces strip along with large sections of New Mexico and California. The mission was an empty gesture, designed largely to pacify those in Washington who insisted on diplomacy before war. Predictably, officials in Mexico City refused to receive Slidell. In preparation for the assumed failure of the negotiations, Polk preemptively sent a four-thousand-man army under General Zachary Taylor to Corpus Christi, Texas, just northeast of the Nueces River. Upon word of Slidell’s rebuff in January 1846, Polk ordered Taylor to cross into the disputed territory. The president hoped that this show of force would push Mexico to sell California in order to avoid a war. Unfortunately, he badly misread the situation. After losing Texas, the Mexican public strongly opposed surrendering any more ground to the United States, which left the shaky government in Mexico City little room to negotiate. On April 24, Mexican cavalrymen attacked a detachment of Taylor’s troops in the disputed territory just north of the Rio Grande, killing eleven U.S. soldiers.

A Declaration of War

It took two weeks for the news to reach Washington. Polk sent a message to Congress on May 11th that summed up the assumptions and intentions of the United States with regard to Mexico.

Instead of this, however, we have been exerting our best efforts to propitiate her good will. Upon the pretext that Texas, a nation as independent as herself, thought proper to unite its destinies with our own, she has affected to believe that we have severed her rightful territory, and in official proclamations and manifestoes has repeatedly threatened to make war upon us for the purpose of reconquering Texas. In the meantime we have tried every effort at reconciliation. The cup of forbearance had been exhausted even before the recent information from the frontier of the Del Norte. But now, after reiterated menaces, Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon the American soil. She has proclaimed that hostilities have commenced, and that the two nations are now at war.^[2]

Polk knew that since hostilities already existed, political dissent would be dangerous—a vote against war became a vote against supporting American soldiers under fire. Congress passed a declaration of war on May 13th. Only a few members of both parties, notably John Quincy Adams and John C. Calhoun, opposed the measure. Upon declaring war in 1846, Congress issued a call for fifty thousand volunteer soldiers. Spurred by promises of adventure and conquest, thousands of eager men flocked to assembly points across the country. However, opposition to “Mr. Polk’s War” grew steadily as more Americans questioned the reasons behind the conflict.

Figure 2. Anti-Catholic sentiment played an important role in the Mexican-American War. The American public widely regarded Roman Catholics as cowardly and vice-ridden, like the clergy in this ca. 1846 lithograph who are shown fleeing the Mexican town of Matamoros accompanied by pretty women and baskets full of alcohol.

The small but vocal anti-slavery faction decried the decision to go to war, arguing that Polk had deliberately provoked hostilities so the United States could annex more slave territory. Illinois representative Abraham Lincoln and other members of Congress issued the Spot Resolutions in which they demanded to know the precise spot on U.S. soil where American blood had been spilled. Many Whigs also denounced the war. Democrats, however, supported Polk’s decision, and volunteers for the army came forward in droves from every part of the country except New England, the seat of abolitionist activity. Enthusiasm for the war was aided by the widely held belief that Mexico was a weak, impoverished country and that the Mexican people, perceived as ignorant, lazy, and controlled by a corrupt Roman Catholic clergy, would be easy to defeat.

The U.S. Goes to War

Mexico and Texas

U.S. military strategists had three main objectives in the Mexican-American War: to take control of northern Mexico, including New Mexico, to seize California, and to capture Mexico City. General Zachary Taylor and his Army of the Center were assigned to accomplish the first goal, and they engaged the Mexican Army in three battles in central Texas and northern Mexico: Palo Alto, Resaca de la Palma, and Monterrey (not to be confused with Monterrey, California). Taylor was victorious in all three battles but treated the Mexican troops with compassion and respect, even helping to perform last rites for some of the Mexican soldiers who died, which earned him widespread praise. For his bravery and his compassion, Taylor quickly became a hero in the eyes of the American people, and Polk appointed him commander of all U.S. forces.

Leading the Army of the South into Mexico was General Winfield Scott. Both Taylor and Scott were potential competitors for the presidency, and believing—correctly—that whoever seized Mexico City would become a hero, Polk assigned Scott the campaign to avoid elevating the more popular Taylor, who was affectionately known as “Old Rough and Ready.” Scott captured Veracruz in March 1847, and moving in a northwesterly direction from there (much as Spanish conquistador Hernán Cortés had done in 1519), he slowly closed in on Mexico City. Every step of the way was a hard-fought victory, however, and Mexican soldiers and civilians both fought bravely to save their land from the American invaders.

New Mexico and California

General Stephen Watts Kearny, commander of the Army of the West, accepted the surrender of Santa Fe, New Mexico, and moved on to California, leaving Colonel Sterling Price in command. Despite Kearny’s assurances that New Mexicans need not fear for their lives or their property, the region’s residents rose in revolt in January 1847 in an effort to drive the Americans away. Although Price managed to put an end to the rebellion, tensions remained high.

Kearny, meanwhile, arrived in California to find it already in American hands through the joint efforts of California settlers, U.S. naval commander John D. Sloat, and John C. Fremont, a former army captain and son-in-law of Missouri senator Thomas Benton. Sloat, at anchor off the coast of Mazatlan, learned that war had begun and quickly set sail for California. He seized the town of Monterey in July 1846, less than a month after a group of American settlers led by William B. Ide had taken control of Sonoma and declared California a republic (which lasted for all of 25 days). A week after the fall of Monterey, the U.S. navy took San Francisco with no resistance. Although some Californios staged a short-lived rebellion in September 1846, many others submitted to the U.S. takeover. Thus, Kearny had little to do other than take command of California as its governor.

The War’s End

Mexico City’s defenders, including young military cadets, fought to the end. According to legend, cadet Juan Escutia’s last act was to save the Mexican flag, and he leapt from the city’s walls with it wrapped around his body. On September 14, 1847, General Scott entered Mexico City’s central plaza, the Zócalo, officially marking the capture of the capital city. While some expansionists called for the U.S. to conquer all of Mexico, the United States negotiated for peace and in 1848, both nations signed the Treaty of Guadalupe Hidalgo, which resulted in the Mexican Cession.

The Mexican Cession

The Treaty of Guadalupe Hidalgo, signed in February 1848, was a triumph for American expansionism under which Mexico ceded nearly half its land to the United States. The Mexican Cession, as the land west of the Rio Grande was called, included the current states of California, New Mexico, Arizona, Nevada, Utah, and portions of Colorado and Wyoming. Mexico also recognized the Rio Grande as the border with the United States. Mexican citizens in the ceded territory were promised U.S. citizenship in the future when the territories they were living in became states, or they could return to Mexico within a year. In exchange, the United States agreed to assume $3.35 million worth of Mexican debts owed to U.S. citizens, paid Mexico $15 million for the loss of its land, and promised to guard the residents of the Mexican Cession against Native American raids.

As extensive as the Mexican Cession was, some argued the United States should not be satisfied yet. The “All Mexico” movement, spearheaded by many eastern Democrats, proposed that the only way to keep peace in the region was to annex all of Mexico to the U.S. Some saw it as an anti-slavery move since the population of Mexico was generally opposed to slavery. However, Southerners who opposed the “All Mexico” movement, such as South Carolina senator John C. Calhoun, did not want to make Mexico’s large mestizo (people of mixed Native and European ancestry), Catholic, non-English-speaking, revolution-prone population part of the United States. They felt that a group so diametrically opposed to White, Protestant, Anglo-American culture would cause too much tension and conflict and might also threaten Anglo racial purity.

The U.S.-Mexican War had an enormous impact on both countries. The American victory helped set the United States on the path to becoming a world power. It elevated Zachary Taylor to the presidency and served as a training ground for many of the Civil War’s future commanders. Most significantly, however, Mexico lost roughly half of its territory. Yet the United States’ victory was not without danger. Ralph Waldo Emerson, an outspoken critic, predicted ominously at the beginning of the conflict, “We will conquer Mexico, but it will be as the man who swallows the arsenic which will bring him down in turn. Mexico will poison us.”^[3] Indeed, the conflict over whether to extend slavery into the newly won territory pushed the United States ever closer to disunion and civil war.

Figure 4. Questions about the balance of free and slave states in the Union became even more fierce after the U.S. acquired these territories from Mexico in the 1848 Treaty of Guadalupe Hidalgo. Map of the Mexican Cession.