Lecture: The Mexican [American] War
United States | Latin America
The Mexican War stands among the most decisive events in Borderlands history — not merely because territory changed hands, but because political worlds collapsed and new ones, uncertain and unfinished, were thrust into being. Mexico would lose a substantial portion of its northern territory, and the United States would become a bicoastal nation. That geographical transformation would shape not only American development but its foreign relations for generations. Yet the war cannot be understood as a sudden eruption. It emerged from disorder — from political fragility, ideological rivalry, and the grinding instability of the frontier.
Mexico entered the conflict already weakened. Politically, the nation was in shambles. Power shifted repeatedly between liberals and conservatives. Revolts flared. Leadership changed with exhausting frequency. Antonio López de Santa Ana — restless, ambitious, and perpetually returning — was compelled to divide his attention among multiple crises, including conflict in Zacatecas. The cost of this instability was catastrophic. Before Mexico ever fired upon United States troops, one estimate suggests that over 100,000 Mexican soldiers had been lost — not solely in combat, but to disease, poor organization, factional bickering, and logistical breakdown. The military was diluted before the war began.
The United States, by contrast, played its hand deliberately. Andrew Jackson had long dreamed of annexing Texas — and even envisioned San Francisco as part of an expanded republic — but left that ambition unrealized. It would fall to James K. Polk, elected in November 1844 after defeating Henry Clay, to carry the banner of Manifest Destiny. Polk ran openly on expansion. The American West, he argued, was destined to be taken from Mexico. He pledged only a single term in office — and within that term, he intended to secure Texas and push the republic coast to coast.
The roots of the Texas crisis lay deeper than diplomacy. Slavery stood at the center. Mexico had abolished slavery under Vicente Guerrero. That decision reverberated across the borderlands. Enslaved people fled southward, seeking asylum in Mexican territory — an alternative corridor of liberation parallel to the Underground Railroad. Even as annexation was debated in the United States Senate, Mexico’s antislavery stance sharpened sectional tensions. On June 6, 1844, the Senate rejected a treaty to annex Texas, leaving the Texas Republic temporarily independent.
Meanwhile, California simmered. Juan Bautista Alvarado had earlier seized power in Alta California, declaring a brief independence before falling back in line with Mexico. Political factions — Californio Norteños and Sureños — competed for dominance. In Santa Fe and New Mexico, French traders moved steadily through commercial networks. Mexico’s frontier citizens felt forgotten by Mexico City. Trade flowed from Boston whalers and merchants up the Alta California coast, and from Asia across the Pacific. Intermarriage created layered alliances. Commerce flourished even as political authority frayed.
In December 1844, Santa Ana was deposed — José Joaquín Herrera became interim President. Within weeks, Santa Ana rebelled again, attacked Puebla, was captured, and by April 1845 was sent into exile in Cuba. Before Polk’s inauguration, outgoing President John Tyler signed the joint resolution offering Texas annexation. Polk took office three days later. By March 31, Mexico’s minister to the United States, Juan Almonte, requested his passports and severed diplomatic relations. The break was profound. Mexico had needed American recognition — now diplomacy gave way to suspicion.
Polk acted swiftly. He ordered General Zachary Taylor to Corpus Christi and dispatched naval forces to the Gulf. On the Pacific coast, British merchants eyed Alta California. Local leaders debated annexation. Mariano Guadalupe Vallejo in the north favored union with the United States. Pío Pico in the south feared American proximity and Protestant influence, preferring alignment with France or Great Britain. Disorganization in Mexico fostered autonomous thinking on the frontier. Protection and prosperity were sought wherever they might be found.
By July 4, 1845, Texas voted for annexation. Diplomatic efforts faltered. Herrera sidestepped American envoy John Slidell. Garrisons called for Herrera’s overthrow, and by December 1845, his government collapsed. Yucatán declared neutrality. Mariano Paredes assumed the presidency. Polk ordered Taylor to the Rio Grande and a blockade of Veracruz, one of Mexico’s principal ports linking Europe, Africa, and the United States. By March 1846, the armies faced each other.
The spark came with the Thornton Affair in April 1846, when U.S. dragoons were attacked east of the Rio Grande. Polk seized the moment. The siege of Fort Brown followed, then the Battle of Palo Alto. On May 11, Polk delivered his war message to Congress. Within days, the United States declared war. Troops moved into the disputed territory.
On the Pacific, events accelerated. John C. Frémont, an army surveyor with presidential ambitions, entered California. On June 14, 1846, the Bear Flag Revolt erupted in the north — ironically on the property of Vallejo, who had supported American annexation. Vallejo attempted conciliation — brandy, wine, stories — but was imprisoned. On July 7, U.S. naval forces occupied Monterey. The Bear Flag Revolt merged with American forces.
Yet California did not fall quietly. José María Flores, Andrés Pico, and other Californio leaders rallied resistance. Though the frontier remained under-resourced, Mexican forces expelled Americans from Los Angeles. Guerrilla warfare ensued. In San Diego, local officials such as Juan Bandini sided with the Americans — Bandini’s daughters helped stitch the first U.S. flag raised in Old Town. Divided loyalties fractured the region.
By August 1846, Santa Ana returned from exile. Stephen Watts Kearny marched 2,000 miles from Texas to California — the longest march in U.S. military history. Contrary to expectation, Apache and Navajo groups did not significantly obstruct him, as many Indigenous communities resented Mexican rule and their inability to pay on the spot. Kearny’s men were ill-equipped — some without shoes, some riding donkeys — yet pressed on.
Battles multiplied: Monterrey in September — Dominguez Rancho in Los Angeles — Tampico seized by the U.S. Navy, and Saltillo occupied. None in Alta California proved decisive at first — skirmishes, guerrilla engagements, rapid reversals. The most famous came on December 6, 1846, at San Pasqual. Kearny, descending upon Californio lancers armed with ashwood lances, underestimated them. Wet gunpowder hampered American fire. In a canyon trap, lances outmatched sabers. Casualties mounted, and Kearny himself was wounded. Andrés Pico prevailed tactically before retreating north.
By early 1847, American momentum intensified. The Taos Revolt erupted. Charles Bent was killed. Veracruz fell after bombardment. Santa Anna again assumed the Presidency. Battles at Contreras and elsewhere opened the road to Mexico City. On September 13–14, 1847, the capital fell. Winfield Scott occupied the city. Santa Anna resigned and fled.
In Alta California, the Treaty of Cahuenga ended hostilities in January 1847, preserving certain Californio protections later echoed in the Treaty of Guadalupe Hidalgo. Meanwhile, on January 24, 1848, gold was discovered at Sutter’s Mill — wealth that would finance American expansion and, ultimately, the Civil War.
On February 2, 1848, the Treaty of Guadalupe Hidalgo was signed. It granted former Mexican citizens the option of U.S. citizenship and addressed property rights, though many land grants would be lost amid legal and linguistic barriers. Amendment XI made Native resistance — particularly from the Comanche — the responsibility of the United States, foreshadowing the Indian Wars of the 1850s and beyond. The final battle occurred at Santa Cruz de Rosales in New Mexico. Santa Anna departed again into exile.
By August 1848, U.S. troops withdrew from Mexico. In November, Zachary Taylor was elected president. Mexico was left territorially diminished and politically unstable. The upheavals of the 1850s and 1860s — the rise of Benito Juárez, renewed civil strife — would follow. In the United States, expansion deepened the slavery question. Figures like Juan Seguín found themselves displaced, mistrusted, and navigating courts in a foreign language.
The war had been, in many respects, inevitable. Manifest Destiny provided ideology, and political fragility provided opportunity. The frontier — neglected, commercial, intermarried, divided — became the crucible. What followed reshaped race, law, sovereignty, and memory in the Borderlands. The conflict ended formally in 1848. Its consequences did not.
