5.1 US-Mexico War and more Westward Expansion

By the late 1700s, the Spanish had conquered territories that encompassed present day Texas, New Mexico, Arizona, California, and the southern thirds of Nevada, Utah, and Colorado. As noted in Module 1, they used the mission system to convert the Native Americans, and had incorporated many of these converted peoples into their culture. Any who resisted were quelled violently.

After gaining their independence from Spain, Mexico decided that the best way to control the far northern territory of their newly independent country would be to populate it, so they offered free land with few requirements and invited Anglo settlers and traders into previously restricted territory. Eventually, these largely Anglo-Americans settled in Texas, California, and Santa Fe, in effect creating American colonies there even though they had become Mexican citizens, convert to Catholicism, and repudiate slavery in order to do so. They largely ignored these obligations and set up businesses and trading outposts. Manifest Destiny became an even more popular idea.

Texas, Mexico, and America

The debate over slavery became one of the prime forces behind the Texas Revolution and the resulting 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 to create a buffer between it and the powerful Comanche. New immigrants, mostly from the southern United States, poured into Mexican Texas. Over the next twenty-five years, concerns over growing Anglo influence and possible American designs on the area produced great friction between Mexicans and the former Americans in the area. In 1829, Mexico, hoping to quell both 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 ignored by Americans who often squatted on public lands.¹

In 1834, an internal conflict between federalists and centralists in the Mexican government led to the political ascendency of General Antonio López 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. Anglo settlers in Mexican Texas, or Texians as they called themselves, opposed Santa Anna’s centralizing policies and met in November. They 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 the Mexican government angrily rejected the offer, Texian leaders soon abandoned their fight for the Constitution of 1824 and declared independence on March 2, 1836.² 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 American civilians known as the Runaway Scrape. The confident Santa Anna consistently failed to make adequate defensive preparations, an oversight that eventually led to a surprise attack from the outnumbered Texian army led by Sam Houston on April 21, 1836. 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 killing fleeing and surrendering Mexican soldiers 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.³

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 reignite 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.”⁴ Beyond the anger produced by annexation, the two nations both laid claim over a narrow strip of land between two rivers. Mexico drew the southwestern border of Texas at the Nueces River, but Texans claimed that the border lay roughly 150 miles farther west at the Rio Grande. Neither claim was realistic since the sparsely populated area, known as the Nueces strip, was in fact controlled by Native Americans.

Relatively recent historical scholarship has found that the native peoples and polities truly controlling the contested lands of what is now the American Southwest, what was then the Mexican North, led America and Mexico to each see cause for war.  Americans were sharply critical of Mexico’s “failure” to “dominate” native peoples, while Mexicans saw American interference in the region as a central element of the troubles they faced.  Complicating this, both nations had strong ideas on race, but these were different – and many American racists excluded some or all the peoples of Mexico from whiteness.

At the same time, the powerful native groups in the region described in Section 4.10 and 4.11 above continue to balance the two powers against one another when possible, trade and raid in almost equal measure, and made use of their better knowledge of the scarce resources of the region in a time when neither nation-state had the technology and infrastructure to replace local resources with imported ones.  This, in turn, further inflamed the American perception of Mexico’s inability to “control” people most white Americans refused to see as capable of resisting a “real” European-born polity – while it further convinced Mexico that this native resistance was fueled or aided by America’s interference.  As mentioned earlier, the stretch of conflicts before and during the US-Mexico War has been described by Brian DeLay as “The War of a Thousand Deserts” for these reasons.

Yet these beliefs of the time ran into contrary evidence during and after the war.  Some scholars see the lack of sustained resistance to the American conquest of Mexican territory by the Mexican citizens in the region as a sign of either weariness with the earlier conflicts or the impact of the conflicts precluded the mobilization of resources needed for such resistance.  And the Americans would find themselves no better able to “control” the native peoples they had criticized Mexico’s attempts at control over, at least until railroad and communication technologies improved enough, and spread far enough, to negate the native people’s advantages in local knowledge and means of making war.

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 the lands of California onto the bargaining table as well. Unfortunately, he badly misread the situation. After losing Texas, the Mexican public strongly opposed surrendering any more ground to the United States. 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 in the disputed territory just north of the Rio Grande, killing eleven U.S. soldiers.

The cagey 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 13. 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 abroad, thousands of eager men flocked to assembly points across the country.⁶ However, opposition to “Mr. Polk’s War” soon grew.

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 viciously debated the news. Volunteers found that war was not as they expected. Disease killed seven times as many American soldiers as combat.⁷ 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.

The United States gained lands that would become the future states of California, Utah, and Nevada; most of Arizona; and parts of New Mexico, Colorado, and Wyoming. Mexican officials would also have to surrender their claims to Texas and recognize the Rio Grande as its southern boundary. The United States offered $15 million for all of it. With American soldiers occupying their capital, Mexican leaders had no choice but to sign.

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, the famed gold deposits of California, and the Rocky Mountains. This postwar migration built earlier paths dating back to the 1820s, when the lucrative Santa Fe trade enticed merchants to New Mexico and generous land grants 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. 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.”⁸ Indeed, the conflict over whether to extend slavery into the newly won territory pushed the nation ever closer to disunion and civil war.

1. David Reimers, Other Immigrants: The Global Origins of the American People (New York: New York University Press, 2005), 27.
2. H. P. N. Gammel, ed., The Laws of Texas, 1822–1897, Vol. 1 (Austin, TX: Gammel, 1898), 1063, https://texashistory.unt.edu/ark:/67531/metapth5872/m1/1071/
3. Randolph B. Campbell, An Empire for Slavery: The Peculiar Institution in Texas, 1821–1865 (Baton Rouge: LSU Press, 1989).
4. Quoted in The Annual Register, or, a View of the History and Politics of the Year 1846, Volume 88 (Washington, DC: Rivington, 1847), 377.
5. James K. Polk, “President Polk’s Mexican War Message,” quoted in The Statesmen’s Manual: The Addresses and Messages of the Presidents of the United States, Inaugural, Annual, and Special, from 1789 to 1846: With a Memoir of Each of the Presidents and a History of Their Administrations; Also the Constitution of the United States, and a Selection of Important Documents and Statistical Information, Vol. 2 (New York: Walker, 1847), 1489.
6. Amy S. Greenberg, Manifest Manhood and the Antebellum American Empire (Cambridge, UK: Cambridge University Press, 2005).
7. James M. McCaffrey, Army of Manifest Destiny: The American Soldier in the Mexican War, 1846–1848 (New York: New York University Press, 1992), 53.
8. Ralph Waldo Emerson, quoted in James McPherson, Battle Cry of Freedom: The Civil War Era (New York: Oxford University Press, 1988), 51.