Mexican-American War

The Mexican-American War (1846–48) achieved U.S. expansionist goals by adding more than one million square miles to the United States—the present-day states of New Mexico , Arizona , and California and portions of Colorado , Nevada , Wyoming , and Utah . Mexico's defeat plunged the nation into political and economic upheaval for many of the remaining years of the nineteenth century.

The annexation (adding to the nation) of Texas to the United States in 1845 was the main cause of the Mexican-American War. Texas had won its independence from Mexico in the Texas Revolution (1835–36). During this conflict, Texas received aid and soldiers from the United States. Mexican officials blamed their own defeat on this U.S. assistance to Texas. Mexico refused to recognize its independence, believing that someday Texas would return to the Mexican nation—but only if the United States did not annex it. In 1843, President Antonio López de Santa Anna (1794–1876) of Mexico warned that a U.S. annexation of Texas would be the same as declaring war against Mexico.

Annexation of Texas

The United States recognized Texan independence in 1837. Texas sought annexation by the United States, and many Americans wanted to annex the territory, even if it meant war. Until 1845, the annexation had been successfully blocked by antislavery forces who feared the addition of another slave state to the Union.

In 1844, Texas Republic president Sam Houston (1793–1863) negotiated with the U.S. government about the annexation. A treaty was proposed to the Senate, but it was rejected. Meanwhile, the Democratic Party convention nominated an ardent expansionist, James K. Polk (1795–1849; served 1845–49), for president. In its campaign platform, the Democratic Party advocated the annexation of Texas and the acquisition of Oregon. When Polk won the election, outgoing president John Tyler (1790–1862; served 1841–45) and Congress passed a resolution offering to annex Texas, which was signed three days before Polk took office. Mexico promptly broke off diplomatic relations with the United States.

Texas formally entered the Union in December 1845, but an explosive question remained to be settled with Mexico about Texas's boundary. President Polk supported Texas's questionable claim that the Rio Grande River was its southwestern frontier; Mexico claimed the border was at the Nueces River farther north. In June, Polk ordered Brigadier General Zachary Taylor (1784–1850) to move his forces into the disputed area.

In the borderlands

By July 1845, Taylor had established a base on the south bank of the Nueces near Corpus Christi. In November, upon learning that the Mexican government was prepared to discuss the boundary issue, Polk sent Louisiana politician John Slidell (1793–1871) to Mexico with instructions to discuss the border with Texas and two other outstanding issues: the purchase of California and the purchase of New Mexico Territory (made up of parts of present-day New Mexico, Arizona, Colorado, and Nevada). Slidell was authorized to offer $25 million for California and $5 million for New Mexico.

Slidell reached Mexico City on December 6, 1845. The Mexican government, in response to growing Mexican opposition to negotiations with the Americans, refused to receive him. On January 13, 1846, Polk ordered Taylor to advance through the disputed territory, from the Nueces River down to the Rio Grande. Meanwhile, a new Mexican government came into power that reaffirmed Mexico's claim to Texas and refused to receive Slidell.

The battle begins

As Polk and Congress prepared to make a formal declaration of war against Mexico, news arrived from Taylor that a large Mexican force had crossed the Rio Grande and surrounded a small unit of U.S. soldiers; eleven Americans were killed and the rest were wounded or captured. A declaration of war was issued on May 13, 1846. Already, Taylor had fought and won the battles of Palo Alto and Resaca de la Palma, and on May 18 he occupied the Mexican town of Matamoros.

The U.S. declaration of war authorized a call-up of 50,000 volunteers and more than doubled the strength of the regular army from 7,200 to 15,540. The undisciplined volunteers were at times troublesome and did not always represent the United States well. A small portion of the young men murdered, robbed, and raped the Mexican people throughout the war in Mexico.

Full-scale war

Polk's initial strategy was to occupy Mexico's northern provinces, blockade Mexican ports, and conquer New Mexico and California. By September, Taylor's army had taken Monterrey in northern Mexico, and by January 1847 American forces in the West had secured New Mexico and California. Although successful militarily, the strategy failed to bring Mexico to terms; in order to do so, Polk decided to shift the major military effort from the north to the heart of the country. The plan called for Major General Winfield Scott (1786–1866) to take Veracruz, a state in eastern Mexico. From there, he could march through the mountains and capture Mexico's capital, Mexico City.

American forces entered Veracruz on March 29, 1847, following a week-long land and naval bombardment (constant shooting with large guns and cannons). On April 8, Scott's army set out along the National Road for the Valley of Mexico, in central Mexico. Mexico once again had a new government and General Santa Anna had once again taken over the presidency. In September, he had taken an army north to oppose Taylor but had been defeated at Buena Vista in February 1847. Having returned to central Mexico, he now prepared to drive back Scott's invasion.

On April 18, Scott defeated Santa Anna at Cerro Gordo, a mountain pass near Veracruz, and on May 15 he reached the city of Puebla, only seventy-five miles from Mexico City. After a delay, Scott resumed the advance, again defeating Santa Anna's forces at Contreras, Churubusco, Molino del Rey, and Chapultepec. He entered the capital in mid-September. Santa Anna fled the country, and the new Mexican president informed Polk that he was prepared to negotiate.

The results of the war

The U.S. Senate ratified the Treaty of Guadalupe Hidalgo on March 10, 1848, bringing an official end to the war. According to the treaty, Mexico relinquished New Mexico and California, an addition of 500,000 square miles to U.S. territory. The U.S.-Mexico boundary was established in the middle of the Rio Grande. In return, the United States paid Mexico $15 million. During negotiations, U.S. leaders assumed an attitude of moral superiority. They viewed the forcible incorporation of almost one-half of Mexico's national territory as their divine right, fulfilling the Manifest Destiny , or divine right to expand, of the United States.

The Treaty of Guadalupe Hidalgo looms large in the history of Mexico. After the war, Mexico experienced decades of political turmoil. Partly because of the loss of valuable territory, the treaty ensured that Mexico would remain an underdeveloped country well into the twentieth century. Mexican historians and politicians view this treaty as a bitter lesson in U.S. aggression.

The war with Mexico was one of the deadliest conflicts in U.S. history in terms of deaths per thousands of men who served. Of more than one hundred thousand U.S. soldiers, sailors, and marines who fought in the war, about fifteen hundred were killed in action, and another eleven thousand died from diseases and wounds.