Before the debate over slavery in the West reached a national level, the issue became one of the prime forces behind the Texas revolution and that republic’s annexation to the United States. After gaining its independence from Spain in 1821 Mexico hoped to attract new settlers to its northern areas in order to create a buffer between it and the expanding western populations of the United States. New immigrants, mostly from the southern United States, poured into Texas. Over the next twenty-five years, concerns over growing Anglo influence and possible American designs on Texas produced great friction between Mexican and American populations. In 1829, Mexico, hoping to quell anger and immigration, outlawed slavery and required all new immigrants to convert to Catholicism. American immigrants, eager to expand their agricultural fortunes, largely ignored these requirements. In response, Mexican authorities closed their territory to any new immigration in 1830- a prohibition roundly elided by Americans who often squatted on public lands.

In 1834, an internal conflict between federalists and centralists in the Mexican government led to the political ascendency of General Antonio Lopez de Santa Anna. Santa Anna, Governing as a dictator, repudiated the federalist Constitution of 1824, pursued a policy of authoritarian central control, and crushed several revolts throughout Mexico prompted by his coup. Texian settlers opposed Santa Anna’s centralizing policies and met in November after issued a statement of purpose that emphasized their commitment to the Constitution of 1824 and declared Texas to be a separate state within Mexico. After angry Mexican rejection of the offer, Texian leaders soon abandoned their fight for the Constitution of 1824 and declared independence on March 2, 1836. The Texas Revolution of 1835-1836 was a successful secessionist movement in the northern district of the Mexican state of Coahuila y Tejas that resulted in an independent Republic of Texas.

At the Alamo and Goliad, Santa Anna crushed smaller rebel forces and massacred hundreds of Texian prisoners. The Mexican army pursued the retreating Texian army deep into East Texas, spurring a mass panic and evacuation by Anglo civilians known as the “Runaway Scrape.” Santa Anna consistently failed to make adequate defensive preparations and was eventually caught by surprise on April 21, 1836 by an attack from the outnumbered Texian army led by Sam Houston. The battle of San Jacinto lasted only eighteen minutes and resulted in a decisive victory for the Texians, who retaliated for previous Mexican atrocities by continuing to kill fleeing and surrendering Mexican troops for hours after the initial assault. Santa Anna was captured in the aftermath and compelled to sign the Treaty of Velasco on May 14, 1836, by which he agreed to withdraw his army from Texas and acknowledged Texas independence. Although a new Mexican government never recognized the Republic of Texas, the United States and several other nations gave the new country diplomatic recognition.

Texas annexation had remained a political landmine since the Republic declared independence from Mexico in 1836. American politicians feared that adding Texas to the Union would provoke a war with Mexico and re-ignite sectional tensions by throwing off the balance between free and slave states. However, after his expulsion from the Whig party, President John Tyler saw Texas statehood as the key to saving his political career. In 1842, he began work on opening annexation to national debate. Harnessing public outcry over the issue, Democrat James K. Polk rose from virtual obscurity to win the presidential election of 1844.  Polk and his party campaigned on promises of westward expansion, with eyes toward Texas, Oregon, and California.  In the final days of his presidency, Tyler at last extended an official offer to Texas on March 3, 1845. The republic accepted on July 4, becoming the twenty-eighth state.

Mexico denounced annexation as “an act of aggression, the most unjust which can be found recorded in the annals of modern history.” However, perhaps the most important point of conflict between Mexico and the United States was a narrow strip of land to which both countries now laid claim. While Mexico drew the southwestern border of Texas at the Nueces River, Texans had claimed that the border lay roughly 150 miles further west at the Rio Grande. Neither claim was realistic. The sparsely populated area, known as the Nueces strip, was in fact controlled by independent Indian tribes.

In November of 1845, President Polk secretly dispatched John Slidell to Mexico City in order to attempt a purchase of 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. Earlier that year, Polk had also sent a 4,000 man army under General Zachary Taylor to Corpus Christi, Texas; just northeast of the Nueces River. Upon word of Slidell’s refusal in January 1846, Polk ordered Taylor to cross into the disputed territory. The President hoped that this show of force would push the lands of California onto the bargaining table as well. He badly misread the situation. After losing Texas, the Mexican public strongly opposed surrendering any more ground to U.S. expansionism. Popular opinion left the shaky government in Mexico City without room to negotiate. On April 24, Mexican cavalrymen attacked a detachment of Taylor’s troops just north of the Rio Grande, killing eleven U.S. soldiers.

It took two weeks for the news to reach Washington. Polk sent a message to Congress on May 11. “We have tried every effort at reconciliation…but now, after reiterated menaces, Mexico…has invaded our territory and shed American blood upon the American soil.” However, with fighting already underway, a vote against war became a vote against supporting American soldiers under fire. Congress passed a declaration of war on May 13. Only a few members of both parties, notably John Quincy Adams and John C. Calhoun, voted against the measure. However, opposition to “Mr. Polk’s War” soon grew widespread. Upon declaring war in 1846, Congress issued a call for 50,000 volunteer soldiers. Spurred by promises of adventure and conquest abroad, thousands of eager men flocked to assembly points across the country.

In the early fall of 1846, the U.S. Army invaded Mexico on multiple fronts and within a year’s time General Winfield Scott’s men took control of Mexico City. However, the city’s fall did not bring an end to the war. Scott’s men occupied Mexico’s capital for over four months while the two countries negotiated. In the United States, the war had been controversial from the beginning. Embedded journalists sent back detailed reports from the front lines, and a divided press spun and debated the news viciously. Volunteers found that the real experience of war was not as they expected. Disease killed seven times as many American soldiers as combat did. Harsh discipline, conflict within the ranks, and violent clashes with civilians led soldiers to desert in huge numbers. Peace finally came on February 2, 1848 with the signing of the Treaty of Guadalupe Hidalgo.

“General Scott’s entrance into Mexico.” Lithograph. 1851. Originally published in George Wilkins Kendall & Carl Nebel, The War between the United States and Mexico Illustrated, Embracing Pictorial Drawings of all the Principal Conflicts (New York: D. Appleton), 1851. Wikimedia.

The new American Southwest attracted a diverse group of entrepreneurs and settlers to the commercial towns of New Mexico, the fertile lands of eastern Texas, and the famed gold deposits of California and the Rocky Mountain chains. This postwar migration built upon migration to the region dating back to the 1820s, when the lucrative Santa Fe trade enticed merchants to New Mexico and generous land grant opportunities brought numerous settlers to Texas. The Gadsden Purchase of 1854 further added to American gains north of Mexico.

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, 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 predicted ominously at the beginning of the war that, “Mexico will poison us.” Indeed, the conflict over whether or not to extend slavery into the newly won territory pushed the nation ever closer to disunion and civil war.