The Mexican-American War

by Drew VandeCreek, Director of Digital Scholarship, Northern Illinois University Libraries

Origins of the Mexican-American War

The United States' armed conflict with Mexico largely emerged from Americans' eagerness to expand their nation westward to the Pacific Ocean. As American trappers and settlers poured across the Great Plains, many began to resent the fact that lands to the south and west of the Louisiana Purchase tract remained territories of Mexico, which had freed itself from Spanish colonial control in 1821. Americans' persistent attempts to settle these lands led to conflict with the Mexican government and, eventually, war.

Storming of Chapultapec - Putnam's AttackThe Mexican Republic had welcomed Americans to settle in their northern territory of Texas in the 1820s, but after a decade it became plain that the Americans disliked Mexican rule. In 1835 the American settlers revolted against Mexico and, in the following year, established their own Republic of Texas. Many Americans immediately began to demand that their nation make Texas a part of the United States. The Mexican government warned that this would mean war.

In 1844 American elected James K. Polk as the nation's new president. Polk had campaigned on the issue of national expansion, calling for the annexation of Texas, Mexican California, and the Oregon Territory that the United States and Great Britain had occupied jointly since 1818. Just before leaving office in early 1845 President John Tyler, a Virginian seeking to provide a new area into which slavery might expand, secured a joint resolution from Congress annexing Texas to the United States. Mexico responded by breaking off diplomatic relations.

Upon taking office President Polk immediately turned to the acquisition of Mexico's northern territories. He first instructed his minister to Mexico to negotiate for the purchase of the territories, but this proposal sparked a wave of indignation and nationalist fervor in Mexico, and the minister left Mexico after only a few months.

Angry that Mexico had rebuffed his offer, Polk sent U.S. troops under the command of General Zachary Taylor to the Rio Grande River in January of 1846. Mexican officials believed that the Texas-Mexico frontier stood one hundred miles to the north, at the Nueces River, and interpreted Polk's move as a deliberate provocation. Mexican troops quickly arrived at the Rio Grande as well, and skirmishes broke out between the two forces. Polk leaped to argue that "Mexico… has invaded our territory and shed American blood upon American soil." Congress quickly provided him with a declaration of war.

In 1845 an American editor wrote that the American annexation of Texas represented the "fulfillment of our manifest destiny to overspread the continent allotted by Providence for the free development of our yearly multiplying millions." By 1846 newspapers across the country had appropriated the term “manifest destiny” in their attempts to show that God intended the American nation to stretch from Atlantic to Pacific.

The Mexican-American War: Military Campaigns

American military forces took up several major campaigns in the course of the Mexican War. Polk began his prosecution of the war in June of 1846 by ordering American forces farther into Mexican territory. He directed Taylor and his men to push southward from the Rio Grande into central Mexico. At the same time Brigadier General Stephen Kearney led a small force overland from Fort Leavenworth, Kansas to seize Santa Fe and the New Mexico territory. Finding little opposition in New Mexico, Kearney marched west to California. There he met up with an American naval force and eventually secured the work of the Bear Flag Revolt, in which American settlers had already declared their independence from Mexico. Finally, a third force under the command of General John E. Wool marched from San Antonio, Texas to the Mexican city of Chihuahua.

General Taylor's force participated in the first significant engagement of the conflict at Monterrey in September of 1846. Taylor boldly divided his force and took the city on September 24. After negotiations, the two sides agreed upon an eight-week armistice during which each general would correspond with his government and await further orders.

President Polk had hoped that a set of quick American victories would compel the Mexican government to bargain away their northern territories. But the Mexicans gave no evidence of capitulating. The president ordered General Taylor to resume operations. But he had decided against marching across the rugged central Mexican terrain to Mexico City. Instead, he ordered Taylor to send his regulars to Tampico on the Gulf of Mexico, where they would come under the command of General Winfield Scott and proceed toward Mexico City from the east. Wool was ordered to abandon his march on Chihuahua in order to reinforce Taylor's depleted commands. A new force, led by Colonel Alexander Doniphan, left Santa Fe and proceeded toward Chihuahua.

While the American forces in central Mexico changed strategies, Mexicans in California and New Mexico struck back in attempts to throw off American occupation. In California the arrival of Kearney's force enabled Americans to defeat Mexican Californios at the Battle of San Gabriel and occupy Los Angeles, effectively ending the conflict there. In New Mexico, American troops under the command of General Sterling Price defeated Mexican rebels at Taos after they had killed officials of the new American government.

The Battle of Buena VistaMexican forces counterattacked in February of 1847 at Buena Vista (above), where they confronted the remainder of Taylor's army and Wool's reinforcements. After using his numerical superiority to batter Taylor's Americans, General Antonio Lopez de Santa Anna inexplicably retreated from the field, allowing Taylor to claim victory.

Winfield Scott's new force opened the final phase of the Mexican-American War when they landed unopposed at Veracruz and occupied the city. As his army marched westward, Scott learned that Santa Anna's forces had dug in at a mountain pass called Cerro Gordo, effectively blocking their path to Mexico City. Advance scouts including Captain Robert E. Lee of Virginia discovered a path around Santa Anna's right flank that allowed American soldiers to circle behind and surprise the enemy. Other American units had succeeded in dragging several artillery pieces to a high point that overlooked the Mexican fortifications. On April 18 Scott's army routed the Mexicans from their positions and cleared the way to the Mexican capital.

After this victory Scott spent three months refitting his army in the Mexican city of Puebla. Many of his soldiers, at the end of their one-year enlistments, returned to their homes. Lacking enough soldiers to continue fighting, Scott waited for reinforcements. Wracked by confusion and retreating toward their capital, the Mexican Army failed to attack Scott's depleted corps.

In August of 1847 Scott turned his rebuilt Army toward Mexico City. Attacking from the south in order to avoid a heavily armed fortress blocking his path, Scott won victories at Contreras and Churubusco. Aware that his army was shrinking rapidly, largely due to the toll taken by subtropical diseases, Scott pressed for a quick conclusion to the fighting. On the morning of September 13, American forces took Chapultepec Castle on the capital's western flank. By that evening they had arrived at the gates to the city, only to find that Santa Anna's forces had evacuated to fight another day.

In the following months Mexican officials carried out a guerilla war, attacking American supply lines. The day after Scott's seizure of Mexico City (above), Mexican forces laid siege to the American supply depot and hospitals at Puebla. Hoping to force Scott to abandon Mexico City in order to secure his supply base, Santa Anna's remaining troops soon joined the siege. Although Scott had been unable to correspond with Washington due to his insecure supply and communication lines, Polk had wisely sent reinforcements to his commander. These forces arrived at Puebla on October 12 and broke the siege, effectively ending the Mexican-American War.

The Mexican-American War: Illinois' Role

States nearer to the Mexican frontier often provided more volunteers for the Mexican War than states a greater distance from the conflict. Nevertheless, Illinois filled the ranks with more than her quota of soldiers. On May 25, 1846 Illinois Governor Thomas Ford issued a proclamation calling for the enlistment of three regiments of infantry and named Alton, on the Mississippi River just north of St. Louis, as their rendezvous point.

The vast majority of these troops enlisted for twelve months' service, which led the Governor to make a second call for troops in April of 1847. The state raised two more regiments of men. In total, the State of Illinois provided 6,123 men to the Mexican War. Of these, 86 were killed and 160 wounded. Twelve of the wounded later died of their injuries.

Illinois troops participated in the Battle of Buena Vista, and the state's Third and Fourth regiments won distinction at Vera Cruz and Cerro Gordo (below). These regiments also participated in General Winfield Scott's march into Mexico City.


After the conclusion of the Mexican-American war, the American diplomat Nicholas Trist negotiated the treaty of Guadalupe-Hidalgo, in which the United States claimed over 500,000 square miles of new territory. These lands included Texas, as well as the Mexican territories of New Mexico and Upper California. Eventually they would become the American states of California, Arizona, and New Mexico, and comprise significant parts of Utah, Colorado, Nevada, and Wyoming. For this territory Mexico received $15 million, as well as $3,250,000 to settle American citizens' claims against the Mexican government. The United States Senate ratified the treaty on March 10, 1848. The Mexican Congress approved it on May 25.

The new territories acquired in the Mexican-American War quickly stoked the flames of sectional controversy in American national politics. Even as the fighting continued, northerners partial to free labor and southerners seeking the expansion of slavery began to quarrel over the fate of the impending acquisitions. In August of 1846 the Pennsylvania Democratic Congressman David Wilmot introduced his Proviso, which sought to prohibit the introduction of slavery into any territory gained by the war. Northern Democrats supported the Wilmot Proviso because it allowed them to support the popular war without advancing the cause of slavery's expansion. Southerners reacted angrily to the Proviso, declaring that it represented a northern conspiracy against their interests. Although Congress defeated the Proviso in 1846, it reappeared again in bills to supply troops and conclude the war. Although it never became law, Wilmot's proposal quickly split the Democratic Party into northern and southern wings and paved the way to Civil War.