Mexican-American War

In 1821 Mexico won its independence from Spain. It attempted to control the territory in its northern regions, which included the area of Texas. Mexico followed Spain's earlier practice of encouraging American citizens to settle in the region. Farming outside the United States exempted them from tariffs and taxes and allowed slavery, even after Mexico outlawed slavery in 1829.

In 1821 Spain had granted American businessman Moses Austin large areas of land to colonize. Following his death, his son, Stephen Austin, undertook colonizing these land grants in 1823. The area was then governed by the newly independent Mexico. By 1830, however, Mexico had grown concerned about the large number of Anglos immigrating to the Texas area. It outlawed any new colonizing and applied the tariffs and taxes it had formerly suspended. Conflicts resulted between the Mexicans and Anglos, which developed into a war in 1832. Then in 1836 Texas declared itself independent, forming the Republic of Texas, and was recognized by the United States. The Mexican government refused to recognize the Lone Star Republic, as it was also called because its flag sported a single star.

In 1769 Spain controlled Alta California, the land that is now California, Nevada, and northern Arizona. Spain encouraged Spanish colonists and missionaries to colonize the region and block English and French expansion west. The Franciscans built 21 missions and converted Native Americans who worked on the missions. These indigenous people were often treated much like enslaved persons. These missions became the chief support system for Spanish social, ranch, and agricultural life. Located near the missions were ranchos and communities of pueblos. A rancho was a large area of land allocated by a Spanish government grant for raising beef cattle, horses, or sheep. A pueblo is a Native American settlement with homes made of adobe or stone. The Spanish brought new diseases with them to Alta California, which led to thousands of Native Americans dying from European diseases they were not immune to.

After Mexico gained independence from Spain, the Mexican government seized the missions from the Franciscans and divided their lands into more vast ranchos. Meanwhile during this period, Anglo influence in Alta California steadily grew. Merchant ships from the East Coast traded with settlers up and down the coast. Anglo settlers continued to immigrate to Mexican-held California. Then in 1841 the first wagon train left Missouri with settlers for California and the Sacramento Valley. In 1846, just before the outbreak of the Mexican-American War, Anglo settlers in the valley declared their area of California to be an independent republic. Their rebellion, called the Bear Flag Revolt, was brief but demonstrated the American people's support for the expansion of the United States to the Pacific Ocean.

America's belief in its Manifest Destiny was the root cause of the Mexican-American War. President James Polk (1845–49) had campaigned on and advocated the philosophical belief that by divine providence U.S. territory should extend southward, northward, and westward to the Pacific Ocean. During Polk's presidency, expansion events occurred rapidly. In 1845 the United States annexed Texas. At the end of the year, the U.S. Congress voted to admit it as the 28th state of the Union, and it became known as the Lone Star State. However, Texas's southern boundary with Mexico was still in dispute.

Mexico argued the Texas border was the Nueces River; the United States claimed the border was the more southerly Rio Grande. Settlement of the dispute was hampered by changing leadership within the Mexican government between 1845 and 1846. In autumn 1845 Polk sent his close political ally John Slidell to secretly negotiate with the Mexicans. Slidell was to resolve the disputed border, negotiate the U.S. purchase of New Mexico and California, and settle previous U.S. claims against Mexico. The Mexican government refused to meet with Slidell. In January 1846 Polk responded by sending U.S. troops under command of General Zachary Taylor to the contested land between the Nueces River and the Rio Grande.

In early May, Polk was writing a message to Congress, urging war with Mexico, when he received word that Mexican troops, after crossing the Rio Grande, had engaged with the U.S. troops under Taylor's command. Some 16 Americans were injured or killed. Polk rewrote his message to Congress, claiming that Mexico had entered U.S. territory by force and "shed American blood on American soil." Congress declared war on May 13, 1846. Politically, the Democrats were in favor of the war; the Whigs, an evolution of the National Republican Party and conservative political opponents of the Democratic-Republicans, were not. Opposition to the war also arose from American abolitionists. They viewed the war as a ploy to increase slave states with the Mexican territory Polk intended to acquire. Later events in the 19th century bore out the accuracy of the abolitionists' concerns.

With war declared, American troops ended up invading Mexico and Mexican-held territories. For the next two years, American troops occupied what are now California and New Mexico, northern Mexico, and eventually even the Mexican capital, Mexico City, in September 1847. The capture of Mexico City marked the end of military fighting. In the 1848 Treaty of Guadalupe Hidalgo, the United States gained from Mexico a huge territory over a half million miles square. Today that area is divided across nine states spanning the American Southwest.

The Treaty of Guadalupe Hidalgo, signed in February 1848, ceded to the United States the sprawling Mexican territories north of the Rio Grande, including Alta California. The treaty excluded only a small area of Mexican land that the United States purchased later, in 1853. That area is now in southern New Mexico and Arizona.

The boundaries of the United States west of the Mississippi were now defined by the 49th parallel to the north, established by treaty with Britain, and the Rio Grande to the south, established by the Treaty of Guadalupe Hidalgo. In addition to the boundary terms of the latter agreement, the United States paid Mexico $15 million. The treaty also extended U.S. protection to the Mexican people in the newly acquired lands. Those Mexicans who stayed on their lands could keep them and become U.S. citizens. In practice, however, this stipulation became complicated because the new lands first became territories of the United States rather than states. As a result, the federal government did not protect Mexican land ownership as fully as it had promised it would.

The popular doctrine of America's Manifest Destiny had been fulfilled. U.S. territory had been expanded to the Pacific Ocean as per the doctrine, and settlers continued to pour into the newly acquired areas. Settlers from the East migrated to the vast new lands, establishing communities of farmers and ranchers. Later, the federal Homestead Act of 1862 encouraged settlers to migrate west in even larger numbers. This immense territory the United States acquired through the treaty eventually formed part or all of California, Texas, Nevada, Utah, Arizona, New Mexico, Colorado, Wyoming, and Oklahoma. In addition, Zachary Taylor, the commander sent by President Polk to the disputed southern border, became a hero. He became president of the United States in 1849.

However, a new problem was brewing in the newly acquired territories. As the prospect of statehood for new territories loomed, so did the question of slavery within those new states. Would the new states be slave or free states? Attempts in Congress to pass a law banning slavery in the territories had led to bitter debate between the North and South. Twice the law failed to pass in the mid-1840s. The Compromise of 1850 eventually settled the question of slavery in the new territories. Nevertheless, the debate over slavery increased in intensity, and regional antagonism grew. In 1861, just over a decade from the signing of the treaty with Mexico, civil war broke out.