Despite rapid population growth, the new republic faced political and financial difficulties. The Mexican government refused to honor Santa Anna’s recognition of independence and fighting continued between Texas and Mexico. In addition, Texas had an enormous debt and no money to repay it. Many Texans still hoped to join the United States. Southerners favored the annexation of Texas, but Northerners objected that Texas would add another slave state to the Union. President Van Buren, like Jackson, did not want to inflame the slavery issue or risk war with Mexico. He put off the question of annexing Texas.

John Tyler, who became the nation’s president in 1841 after the death of William Henry Harrison, supported adding Texas to the Union and persuaded Texas to reapply for annexation. However, the Senate was divided over slavery and failed to ratify the annexation treaty. The situation changed with the 1844 presidential campaign. The feeling of Manifest Destiny was growing. The South favored annexation of Texas while the North demanded control of the Oregon. The Democratic candidate, James K. Polk, supported both actions. Whig Henry Clay initially opposed adding Texas to the Union. When he finally came out for annexation, it lost him votes in the North and the election. After Polk’s victory, supporters of annexation pressed the issue in Congress. They proposed and passed a resolution to annex Texas. On December 29, 1845, Texas officially became a state of the United States.

In the early 1800s, New Mexico was the name of a vast region sandwiched between the Texas and California territories. It included present-day New Mexico, Arizona, Nevada, and Utah, and parts of Colorado and Wyoming. Native American peoples had lived in the area for thousands of years. When Mexico won its independence in 1821, it inherited the New Mexico province from Spain. The Mexicans, however, had little control over the distant province. The inhabitants of New Mexico mostly governed themselves. The Spanish had tried to keep Americans away from Santa Fe, fearing that Americans would want to take over the area. The Mexican government changed this policy, welcoming American traders into New Mexico. It hoped that the trade would boost the economy of the province.

William Becknell, the first American trader to reach Santa Fe, arrived in 1821 with a pack of mules loaded with goods. Becknell sold the merchandise he brought for many times what he would have received for it in St. Louis. Becknell’s route came to be known as the Santa Fe Trail. The trail left the Missouri River near Independence, Missouri, and crossed the prairies to the Arkansas River. It followed the river west toward the Rocky Mountains before turning south into New Mexico Other traders followed Becknell, and the Santa Fe Trail became a busy trade route for hundreds of wagons. Americans brought cloth and firearms, which they exchanged in Santa Fe for silver, furs, and mules. As trade with New Mexico increased, Americans began settling in the region. In the United States, the idea of Manifest Destiny captured the popular imagination, and many people saw New Mexico as territory worth acquiring.

Spanish explorers and missionaries from Mexico had been the first Europeans to settle in California in the 1760s. They began building a string of missions that eventually extended from San Diego to Sonoma. The mission system was a key part of Spain’s plan to colonize California. The Spanish used the missions to convert Native Americans to Christianity. By 1820, California had 21 missions, with about 20,000 Native Americans living in them, and they were generally treated as slaves. After Mexico gained its independence from Spain in 1821, California became a state in the new Mexican nation. At the time only a few hundred Spanish settlers lived in California, but emigrants began arriving from Mexico. The wealthier settlers lived on ranches devoted to raising cattle and horses.

In 1833, the Mexican government passed a law abolishing the missions. The government gave some of the lands to Native Americans and sold the remainder. Mexican settlers bought these lands and built huge properties called ranchos. The Mexican settlers persuaded Native Americans to work their lands and tend their cattle in return for food and shelter. The California ranchos were like the plantations of the South, and the rancheros, ranch owner, treated Native American workers almost like slaves again.

Americans had been visiting California for years. Most arrived on trading or whaling ships, although a few hardy travelers like Smith came overland from the East. Soon more began to arrive. At first, the Mexican authorities welcomed Americans in California. The newcomers included agents for American shipping companies, fur traders from Oregon, and merchants from New Mexico. In the 1840s, families began to arrive in California to settle. They made the long journey from Missouri on the Oregon Trail and then turned south after crossing the Rocky Mountains. Still, by 1845 the American population of California numbered only about 700. Most Americans lived in the Sacramento River valley. Some American travelers wrote glowing reports of California.

Americans began to talk about adding California to the nation. Shippers and manufacturers hoped to build ports on the Pacific coast for trade with China and Japan. Many Americans saw the advantage of extending United States territory to the Pacific. That way the nation would be safely bordered by the sea instead of by a foreign power.

Polk was determined to get California and New Mexico from Mexico. Their possession would guarantee that the United States had clear passage to the Pacific Ocean—an important consideration because the British still occupied part of Oregon. Polk’s main reason, though, involved fulfilling the nation’s Manifest Destiny. Like many Americans, Polk saw California and New Mexico as rightfully belonging to the United States. After Mexico refused to sell California and New Mexico, President Polk plotted to pull the Mexican provinces into the Union through war. He wanted, however, to provoke Mexico into taking military action first. This way, Polk could justify the war to Congress and the American people. Relations between Mexico and the United States had been strained for some years. When the United States annexed Texas in 1845, the situation worsened. Mexico, which had never recognized the independence of Texas, charged that the annexation was illegal.

Another dispute concerned the Texas-Mexico border. The United States insisted that the Rio Grande formed the border. Mexico claimed that the border lay along the Nueces River, 150 miles farther north. Because of this dispute, Mexico had stopped payments to American citizens for losses suffered during Mexico’s war for independence. Polk sent an agent, John Slidell, to Mexico to propose a deal. Slidell was authorized to offer $30 million for California and New Mexico in return for Mexico’s acceptance of the Rio Grande as the Texas boundary. In addition, the United States would take over payment of Mexico’s debts to American citizens.

The Mexican government refused to discuss the offer and announced its intention to reclaim Texas. In response. Polk ordered General Zachary Taylor to march his soldiers across the disputed borderland between the Nueces River and the Rio Grande. Taylor followed the order and built a fort there on his arrival. On April 24, Mexican soldiers attacked a small force of Taylor’s soldiers. Taylor sent the report the president wanted to hear: “Hostilities may now be considered as commenced.” Polk called an emergency meeting of his cabinet, and the cabinet agreed that the attack was grounds for war with Mexico. On May 11, 1846, the president told Congress that Mexico had “invaded our territory and shed American blood upon the American soil.” Congress passed a declaration of war against Mexico.

The American people were divided over the war with Mexico. Polk’s party, the Democrats, generally supported the war. Many Whigs opposed it, calling Polk’s action aggressive and unjust. Northerners accused Democrats of waging the war to spread slavery. Illinois congressman Abraham Lincoln demanded to know the exact spot where the first attack against American troops had occurred. Lincoln, like many who opposed the war, claimed that the spot was clearly in Mexico and that Polk therefore had no grounds for blaming the war on Mexico. Frederick Douglass, an African American leader in the antislavery movement, called the war “disgraceful” and “cruel.” Douglass shared the belief that if the United States expanded into the West, the Southern states would carry slavery into the new territories. Newspapers generally supported the war, and volunteers quickly signed up for military service. As time went on, however, anti-war feeling grew, particularly in the North.

Polk had a three-part plan for the war with Mexico. First, American troops would drive Mexican forces out of the disputed border region in Texas and make the border secure. Second, the United States would seize New Mexico and California. Finally, American forces would take Mexico City, the capital of Mexico. Taylor accomplished the first goal quickly. His army captured Matamoros in May and Monterrey in September of 1846. In February 1847, Taylor defeated the Mexicans again at Buena Vista. The Texas border was secure.

While Taylor made progress in northern Mexico, American forces also advanced farther west. General Stephen Watts Kearny led his troops to New Mexico and California. In the summer of 1846, Kearny led about 1,500 cavalry soldiers along the Santa Fe Trail from Fort Leavenworth to New Mexico. The Mexican governor fled, allowing the Americans to capture New Mexico’s capital, Santa Fe, on August 18, 1846, without firing a shot. Kearny and his army then headed across the deserts of New Mexico and Arizona to California.

In June 1846, a small group of Americans seized Sonoma, north of San Francisco, and proclaimed the independent Republic of California. They called the new country the Bear Flag Republic because their flag showed a bear and a star on a white background. John C. Frémont and mountain man Kit Carson, who were already out West on a military expedition in California, joined the Americans in Sonoma. Though unaware of the outbreak of war with Mexico, Frémont declared he would conquer California. Frémont’s actions outraged many Californios, the Mexicans who lived in California. They might have supported a revolt for local control of government, but they opposed what looked like an attempt by a band of Americans to seize land.

In July 1846, the United States Navy under Commodore John Sloat captured Monterey and San Francisco. Sloat declared California annexed to the United States, and the American flag replaced the Bear Flag in California. Sloat’s fleet sailed for San Diego, carrying Frémont and Carson. The Americans captured San Diego and moved north to Los Angeles. Carson headed east with the news of California’s annexation. On his way, he met and joined Kearny’s force, marching west from Santa Fe. After Sloat’s ships left, many Californios in San Diego rose up in arms against the Americans who had taken over the city. Kearny and his troops arrived during the rebellion. They faced a stiff fight but eventually won. By January 1847, California was fully controlled by the United States.

With their victories in New Mexico and California, the Americans met their first two goals in the war. President Polk then launched the third part of his war plan, an attack on Mexico City. Polk gave the task of capturing Mexico City to General Winfield Scott. In March 1847, Scott’s army landed on the coast of the Gulf of Mexico, near the Mexican port of Veracruz. Scott captured Veracruz after a three-week siege and then set out to march the 300 miles to Mexico City. The Americans had to fight their way toward Mexico City, battling not only the Mexican army but also bands of armed citizens. Scott reached the outskirts of Mexico City with his troops towards the end of August 1847. By mid-September, the Americans had taken Mexico City. The Mexican government surrendered.

The United States lost 1,721 men to battle and more than 11,000 to disease in the war with Mexico. Mexico’s losses were far greater. The war cost the United States nearly $100 million, but here, too, Mexico paid a higher price. The war would cost Mexico half its territory. Peace talks between the United States and Mexico began in January 1848. The Treaty of Guadalupe Hidalgo was signed in February 1848. In the treaty Mexico gave up all claims to Texas and agreed to the Rio Grande as the border between Texas and Mexico. Furthermore, in what was called the Mexican Cession, Mexico ceded—gave—its provinces of California and New Mexico to the United States. In return the United States gave Mexico $15 million. Ironically, that was half of what Polk had offered Mexico for the territories just a few years before.

In 1853, the United States paid Mexico an additional $10 million for the Gadsden Purchase, a strip of land along the southern edge of the present-day states of Arizona and New Mexico. With the Gadsden Purchase, the United States mainland reached its present size. All that remained was to settle the newly acquired territories.