Mexican-American War - Invasion, Conflict, Treaty (2024)

When war broke out, former Mexican president and general Antonio López de Santa Anna (the vanquisher of the Texan forces at the Alamo in 1836) contacted Polk. The U.S. president arranged for a ship to take Santa Anna from his exile in Cuba to Mexico for the purpose of working for peace. Instead of acting for peace, however, on his return, Santa Anna took charge of the Mexican forces.

Following its original plan for the war, the United States sent its army from the Rio Grande, under Taylor, to invade the heart of Mexico while a second force, under Col. Stephen Kearny, was to occupy New Mexico and California. Kearny’s campaign into New Mexico and California encountered little resistance, and the residents of both provinces appeared to accept U.S. occupation with a minimum of resentment. Meanwhile, Taylor’s army fought several battles south of the Rio Grande, captured the important city of Monterrey, and defeated a major Mexican force at the Battle of Buena Vista in February 1847. But Taylor showed no enthusiasm for a major invasion of Mexico, and on several occasions he failed to pursue the Mexicans vigorously after defeating them. In disgust, Polk revised his war strategy. He ordered Gen. Winfield Scott to take an army by sea to Veracruz, capture that key seaport, and march inland to Mexico City. Scott took Veracruz in March after a siege of three weeks and began the march to Mexico City. Despite some Mexican resistance, Scott’s campaign was marked by an unbroken series of victories, and he entered Mexico City on September 14, 1847. The fall of the Mexican capital ended the military phase of the conflict.

Ultimately, infection and disease took many more U.S. casualties than combat did. At least 10,000 troops died of illness, whereas some 1,500 were killed in action or died of battle wounds (estimates of the war’s casualties vary). Poor sanitation contributed to the spread of illness, with volunteers—who were less disciplined in their sanitary practices than regular troops were—dying in greater numbers than the regulars. Yellow fever was particularly virulent, but other diseases—such as measles, mumps, and smallpox—took their toll too, especially on troops from rural environments whose immunities were less developed than those of their urban compatriots.

Treaty of Guadalupe Hidalgo and the war’s legacy

Polk had assigned Nicholas Trist, chief clerk in the State Department, to accompany Scott’s forces and to negotiate a peace treaty. But after a long delay in the formation of a new Mexican government capable of negotiations, Polk grew impatient and recalled Trist. Trist, however, disobeyed his instructions and on February 2, 1848, signed the Treaty of Guadalupe Hidalgo. According to the treaty, which was subsequently ratified by both national congresses, Mexico ceded to the United States nearly all the territory now included in the states of New Mexico, Utah, Nevada, Arizona, California, Texas, and western Colorado for $15 million and U.S. assumption of its citizens’ claims against Mexico.

Zachary Taylor emerged as a national hero and succeeded Polk as president in 1849. The war reopened the slavery-extension issue, which had been largely dormant since the Missouri Compromise. On August 8, 1846, Rep. David Wilmot of Pennsylvania attempted to add an amendment to a treaty appropriations bill. The Wilmot Proviso—banning slavery from any territory acquired from Mexico—was never passed, but it led to acrimonious debate and contributed greatly to the rising sectional antagonism. The status of slavery in the newly acquired lands was eventually settled by the Compromise of 1850, but only after the nation had come perilously close to civil war. When the Civil War came in 1861, many of the most-noteworthy generals on both sides had profited from their battle experience in the Mexican-American War, including Confederate Generals Robert E. Lee, Thomas (“Stonewall”) Jackson, James Longstreet, George Pickett, Albert Sidney Johnston, Lewis Armistead, and P.G.T. Beauregard, as well as Union Generals Ulysses S. Grant (who later called the Mexican War “one of the most unjust ever waged by a stronger against a weaker nation”), George Gordon Meade, George H. Thomas, and Joseph Hooker.

In Mexico the war discredited the conservatives but left a stunned and despondent country. It also reinforced the worst stereotypes each country held about the other. Normalization of relations after the war proceeded slowly.