Mexican–American War

Manifest Destiny and Continental Expansion

The Mexican-American War (1846–1848) transformed the United States from an Atlantic republic into a continental nation stretching to the Pacific Ocean. Sparked by the annexation of Texas in 1845 and a border dispute along the Rio Grande, the conflict resulted in the largest territorial acquisition in American history after the Louisiana Purchase. President James K. Polk's vision of "Manifest Destiny"—the belief that American expansion across the continent was both justified and inevitable—drove the nation into a war that would add over 500,000 square miles to the United States.

The Road to War

Texas had declared independence from Mexico in 1836 following the legendary battles at the Alamo and San Jacinto. For nearly a decade, the Republic of Texas existed as an independent nation, and the Daughters of the Republic of Texas preserves this unique heritage today. When Texas accepted annexation as the 28th state on December 29, 1845, Mexico—which had never recognized Texan independence—severed diplomatic relations with the United States. The two nations disputed whether the border lay at the Nueces River (Mexico's position) or the Rio Grande (the American claim). President Polk ordered General Zachary Taylor's army to the Rio Grande in January 1846. On April 25, Mexican cavalry crossed the river and ambushed an American patrol, killing or wounding sixteen soldiers. Polk declared that Mexico had "shed American blood upon American soil," and Congress declared war on May 13, 1846.

Theaters of Operation

The war unfolded across multiple fronts spanning thousands of miles:

The Northern Mexico Campaign under General Zachary Taylor drove south from the Rio Grande. After victories at Palo Alto and Resaca de la Palma in May 1846, Taylor's forces captured Monterrey in September after fierce street fighting. His outnumbered army then achieved a stunning defensive victory at Buena Vista on February 22–23, 1847, repelling a Mexican force three times its size under General Santa Anna.

The California Campaign secured the Pacific coast. Captain John C. Frémont led American settlers in the "Bear Flag Revolt," while Commodore John D. Sloat's Pacific Squadron captured Monterey and San Francisco. Despite a Californio uprising that briefly recaptured Los Angeles, American forces under Commodore Robert Stockton and General Stephen Kearny secured California by January 1847.

The New Mexico Campaign saw General Kearny's "Army of the West" march 900 miles from Fort Leavenworth to capture Santa Fe without a shot on August 18, 1846, securing the Southwest for the United States.

The Mexico City Campaign proved decisive. General Winfield Scott executed America's first major amphibious assault, landing 10,000 troops near Veracruz on March 9, 1847. After a siege forced the port's surrender, Scott's army fought its way inland, defeating Mexican forces at Cerro Gordo, Contreras, Churubusco, Molino del Rey, and Chapultepec before capturing Mexico City on September 14, 1847.

The Training Ground for Civil War Generals

The Mexican-American War served as the crucible for a generation of military leaders who would later face each other across Civil War battlefields. More than 300 future Civil War generals served as junior officers in Mexico, learning lessons in combat, logistics, and leadership that shaped the conflict to come.

Captain Robert E. Lee served on General Scott's staff, conducting daring reconnaissance missions that found routes the Mexicans believed impassable. Lee's experiences under Scott—particularly his emphasis on audacity against superior numbers—would define his Civil War generalship. Lieutenant Ulysses S. Grant served under both Taylor and Scott, absorbing Taylor's unpretentious command style and Scott's bold offensive tactics. At the Battle of Monterrey, Grant delivered critical messages by hanging off the side of his horse to evade enemy snipers—an early display of the determination that would make him Lincoln's indispensable commander.

Other future Civil War luminaries who earned their spurs in Mexico included Thomas "Stonewall" Jackson, George McClellan, George Meade, James Longstreet, P.G.T. Beauregard, Braxton Bragg, Joseph Johnston, Jefferson Davis, and George Pickett. These men fought side by side as brothers in arms, learning each other's strengths and weaknesses—knowledge they would later exploit as enemies.

The Treaty of Guadalupe Hidalgo

The Treaty of Guadalupe Hidalgo, signed on February 2, 1848, formally ended the war. Mexico ceded approximately 525,000 square miles—including present-day California, Nevada, Utah, most of Arizona and New Mexico, and portions of Colorado and Wyoming—in exchange for $15 million. The Rio Grande became the recognized boundary of Texas. The acquired territory would eventually yield incalculable mineral wealth, including the gold that sparked the California Gold Rush just nine days before the treaty was signed.

The Cost and Legacy

Approximately 78,000 American soldiers served in the Mexican-American War. Of these, roughly 13,000 died—the majority from disease rather than combat. The war proved highly controversial, opposed by many Americans including Congressman Abraham Lincoln, who challenged President Polk to identify the exact "spot" where American blood had been shed. Transcendentalist writer Henry David Thoreau went to jail rather than pay taxes supporting the war, penning his influential essay "Civil Disobedience."

Yet the war's military significance was profound. It demonstrated the effectiveness of West Point-trained officers, validated the importance of combined arms operations and amphibious assault, and established patterns of volunteer mobilization that would prove essential in the Civil War. The territory acquired in 1848 also intensified the sectional crisis over slavery's expansion—a conflict that many Mexican War veterans would be called upon to settle by arms barely a decade later.

Tracing Mexican-American War Heritage

Veterans of the Mexican-American War served in various capacities:

• Regular Army: Officers and enlisted men of the United States Army
• Volunteer Regiments: State units raised for federal service, often for limited terms
• Naval Forces: Sailors and Marines of the Pacific and Gulf squadrons
• Texas Rangers: Mounted volunteers who provided reconnaissance and fought guerrilla warfare
• Support Personnel: Surgeons, chaplains, quartermasters, and teamsters

The Republic of Texas—which existed as an independent nation from 1836 to 1845—represents a unique chapter in American history. Veterans of the Texas Revolution and the Republic period, honored by the Daughters of the Republic of Texas, laid the foundation for the conflict that followed. Their struggle for independence at the Alamo, Goliad, and San Jacinto became inseparable from the story of American westward expansion.

Key Dates

Territory Acquired

The Mexican Cession of 1848 included all or part of the following future states:

• California (entire state)
• Nevada (entire state)
• Utah (entire state)
• Arizona (most of the state)
• New Mexico (most of the state)
• Colorado (western portion)
• Wyoming (southwestern corner)