Mexican-American War Buildup
The roots of the Mexican-American War trace back to the Texas Revolution (October 2, 1835–April 21, 1836). After gaining its independence from Spain in 1821, the Mexican government initially encouraged Americans to immigrate to the area that is now Texas. By 1835, the Americans who occupied Texas successfully overthrew Mexican rule in the Texas Revolution.
Following the defeat of his army at the Battle of San Jacinto (April 21, 1836), Mexican President Antonio López de Santa Anna signed the Treaties of Velasco on May 14, 1836. The two treaties granted independence to Texas and established the Rio Grande River as the border between Texas and Mexico.
For the next nine years, the status of Texas and its border remained in flux. The Mexican government refused to ratify the Treaties of Velasco because President Santa Anna signed them while the Texans were holding him as a prisoner of war. On March 1, 1845, the United States Congress passed a joint resolution annexing the Republic of Texas, and on December 29, 1845, Texas joined the Union as the 28th state. Mexico responded by severing diplomatic relations with the United States.
Texas statehood transformed the border feud between Mexico and Texas into an international dispute between Mexico and the United States. Citing the Treaties of Velasco, American authorities claimed that the southern border of Texas was the Rio Grande River. Mexican officials maintained that the southern extent of Texas was the Nueces River, roughly 150 miles north of the Rio Grande.
U.S. Escalation and Mobilization
As tensions between the two nations mounted, in July 1845 President James K. Polk mobilized about 3,500 soldiers, under the command of General Zachary Taylor, at Corpus Christi, Texas, near the mouth of the Nueces River. Polk also ordered Commodore David Conner to assemble an American squadron in the Gulf of Mexico.
Another source of tension between the two nations was the ownership of land west of Texas. Mexico claimed the territory that now includes west Texas, New Mexico, Arizona, and California.
President Polk firmly believed that the United States was destined to expand its borders to the Pacific Ocean. Expansionist journalists adopted his stance and popularized the concept as America’s Manifest Destiny.
Mexican-American War History
American Forces Advance
In November 1845, less than a year after taking office, Polk attempted to fulfill his expansionist ambitions by offering to purchase the Mexican territories west of Texas, and the land under dispute on the Texas border, for $25 million. When the Mexican government rejected his overtures, Polk ordered Taylor and his troops to cross the Nueces River.
In March 1846, Taylor pushed his forces into the Nueces Strip, the disputed area between the Nueces and the Rio Grande rivers. Under the direction of Captain Joseph K. Mansfield, Taylor’s soldiers built a star-shaped earthwork named Fort Texas on the north banks of the Rio Grande, opposite the Mexican village of Matamoros. Mexican authorities responded by dispatching troops commanded by General Mariano Arista to Matamoros.
The Thornton Affair
On April 25, 1846, a cavalry detachment of 2,000 Mexicans encountered a patrol of 70 U.S. soldiers north of the Rio Grande River. The ensuing engagement, known as the Thornton Affair, resulted in the death of 16 Americans.
Declarations of War
When President Polk learned of the clash, he sent a message to Congress on May 11, proclaiming that “Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon American soil.” Two days later, on May 13, 1846, Congress approved a declaration of war against Mexico. Mexican officials reciprocated by declaring war against the United States on July 7.
With a declaration of war in hand, President Polk was free to pursue his expansionist designs, while simultaneously attempting to settle the more immediate issue of establishing the border between Texas and Mexico. The War Department developed plans to achieve Polk’s aims by engaging the Mexicans on four fronts.
Texas-Northern Mexico Front
Following the Thornton Affair, Mexican General Arista began siege operations against the small American garrison at Fort Texas. General Taylor responded by marching 2,200 soldiers into a position to challenge Arista. Leaving a small contingent behind to continue the siege, Arista moved to accept the challenge. On May 8, 1846, Taylor’s outnumbered army defeated Arista’s force of 3,400 Mexicans at the Battle of Palo Alto, near present-day Brownsville, Texas.
Following the American victory, Arista moved his troops to a nearby strong defensive position in a dry river bed known as Resaca de la Palma. Taylor’s forces routed the Mexican army on May 9, 1846, at the Battle of Resaca de la Palma, forcing them back across the Rio Grande River. During the two days of fighting, Arista lost roughly one-third of his army, compared to fewer than 200 casualties for the Americans.
In mid-May, after the declaration of war, Taylor moved his army across the Rio Grande and occupied Matamoras. From there he launched an offensive into Northern Mexico. By then, the size of his army had swelled to about 6,000 soldiers. Taylor next moved against the Mexican city of Monterrey, defended by roughly 7,000 Mexican soldiers commanded by General Pedro de Ampudia. Hostilities began on September 21, 1846, and following three days of brutal urban combat, Ampudia surrendered on September 24. After the American victory at the Battle of Monterrey, President Polk, and others severely criticized Taylor for his generous terms of surrender, which allowed Ampudia’s army to retreat south.
By the end of 1846, Taylor’s Army of Occupation controlled most of Northern Mexico. After Mexican officials rebuffed Polk’s attempts to reach a negotiated settlement, the War Department decided to strike at the heart of Mexico. Army officials determined that it was impractical to maintain overland supply lines throughout the invasion, so Polk ordered General Winfield Scott to lead an amphibious invasion into Central Mexico.
In early 1847, Scott commandeered roughly 8,000 of Taylor’s army to take part in the invasion. Scott’s action left Taylor with only about 5,000 soldiers, most of whom were volunteers, garrisoned at Saltillo. When General Santa Anna (who had returned from exile and assumed command of the Mexican army) learned of Scott’s plan, he struck Taylor’s army while it was vulnerable.
After marching north from San Luis Potosí, Santa Anna positioned 20,000 Mexican soldiers at Encarnacion, south of Saltillo, on February 20, 1847. Learning of Santa Anna’s approach, Taylor deployed his troops in a strong defensive position at the mountain pass south of Buena Vista on February 21. The next day, Santa Anna’s army attacked after Taylor refused to surrender. Despite being outnumbered nearly four to one, the Americans withstood furious assaults by the Mexican army for the next day and a half. After failing to dislodge Taylor’s army, Santa Anna withdrew on the night of February 23. The American victory at the Battle of Buena Vista was the last major engagement of the Northern Mexico Campaign.
As U.S.-Mexican relations deteriorated following the annexation of Texas, Mexican officials began threatening to expel American immigrants living in the Mexican province of Alta California, which included the present-day State of California. On June 14, 1846, a group of about thirty Americans seized the pueblo of Sonoma, north of San Francisco Bay, declared their independence, and established the California Republic. About ten days later Lieutenant Colonel John C. Frémont, who was leading an exploratory party of roughly sixty American soldiers in the area, joined the rebels, without authorization from the U.S. government. Frémont’s bold, albeit unauthorized action paved the way for President Polk to seize California.
Soon after Congress declared war, U.S. forces swung into action in Alta California. On July 7, 1846, U.S. Marines commanded by Commodore John D. Sloat occupied the coastal city of Monterey, about 125 miles south of San Francisco. Two days later, a contingent of American sailors and marines seized the city of Yerba Buena (present-day San Francisco) without firing a shot. On July 15, 1846, Commodore Robert F. Stockton arrived at Monterey Bay took command of U.S. forces in California, including Frémont’s soldiers and volunteers. Authorities named Frémont commander of the California Battalion of the U.S. Army and the California Republic disbanded to become an American military occupation zone.
By late July, Northern California was under U.S. control. Stockton and Frémont next turned their attention to the south. Frémont’s men sailed to San Diego and arrived there by the end of the month. As Stockton moved toward Los Angeles, Mexican officials withdrew. On August 13, 1846, both American forces occupied the city, leaving nearly all of California under U.S. control.
Mexican citizens living in California, known as the Californios, resisted the American occupation, however. Under the leadership of Captain José María Flores, the Californios fought back. From September 22-30, 1846, they besieged Los Angeles and retook the city. On September 26–27, the Californios defeated a small contingent of American soldiers at the Battle of Chino. October 7, 1846, marked the high-water mark of the Californios’ efforts to reclaim their land when they defeated a force of 200 marines at the Battle of Dominguez Rancho.
In January 1847, Stockton merged his forces with Frémont’s and with soldiers General Stephen W. Kearny had marched into California from New Mexico. With nearly 1,000 soldiers under his command, Stockton urged Flores to surrender. After Flores refused, Stockton’s army won a hard-fought victory at the Battle of Rio San Gabriel on January 8. The Americans won a follow-up encounter the next day at the Battle of La Mesa. With Stockton threatening to hang Flores if he captured him, Flores fled for Mexico on January 11. The next day, Flores’ forces surrendered, leaving Stockton and his army in control of California.
New Mexico Front
At about the same time that Taylor crossed the Rio Grande River, two other American forces moved into Northern Mexico farther to the west. Brigadier General John E. Wool led roughly 2,000 men out of San Antonio on June 5, 1846. His Army of the Center occupied Parras on December 5, and eventually joined General Taylor’s army in time for the Battle of Buena Vista.
Also in June 1846, Colonel Stephen W. Kearny headed southwest out of Fort Leavenworth, in the Kansas Territory, with about 1,650 soldiers. In early August 1846, at the Battle of Cañoncito, a Mexican force commanded by Manuel Armijo failed to halt Kearney’s progress toward Santa Fe, the capital of the Mexican province of Nuevo Mejico. On August 18, Kearney’s Army of the West occupied the important stop on the Santa Fe Trail. He remained at Santa Fe for about five weeks before departing for California on September 18, leading 300 roughly dragoons. Kearny arrived in California in time to assist Commodore Robert F. Stockton in the American victories at the Battle of Rio San Gabriel on January 8, 1847, the Battle of La Mesa on January 9, and the occupation of Los Angeles on January 10.
Central Mexico Front
Despite American successes in Northern Mexico, New Mexico, and California, Mexican officials rebuffed President Polk’s entreaties for a negotiated end to the war. Polk responded by meeting with General Winfield Scott in late 1846 to plan an invasion of Central Mexico. By October, Scott was busy developing plans and assembling an expeditionary army. The bulk of Scott’s force, which rarely numbered over 13,000 troops, comprised roughly 8,000 soldiers that he siphoned from General Zachary Taylor’s army in Northern Mexico.
Siege of Veracruz
After assembling his forces on Lobos Island, in the Gulf of Mexico, Scott sailed for Mexico on March 2, 1847. On March 9, the American “Army of Invasion” began landing operations at the Mexican port city of Veracruz. Scott’s headway was unopposed because General Juan Morales, commander of the garrison at Veracruz kept his 4, 300 soldiers within the safety of the walls surrounding the city. Scott wisely besieged the city rather than assaulting it. After withstanding two weeks of bombardment from American artillery, Morales surrendered Veracruz on March 28, 1847. Scott’s conquest of Veracruz established a foothold on the Mexican coast from which to supply his army as he began his march inland.
Battle of Cerro Gordo
Scott’s ultimate target was Mexico City, roughly 265 miles to the west. Within a week after the fall of Veracruz, Scott began his march toward the Mexican capital. After advancing about sixty-five miles, Scott’s Army of Invasion approached General Santa Anna’s Mexican army’s fortifications at Cerro Gordo, near the city of Xalapa. Despite being outnumbered 12,000 to 8,500, Scott’s forces dislodged the Mexicans at the Battle of Cerro Gordo on April 18, 1847. Scott occupied Xalapa the following day. In mid-May, he pushed on to the city of Pueblo, about 100 miles to the west and eighty-five miles from Mexico City.
Scott’s advance on Mexico City stalled during the summer because of circumstances beyond his control. Sickness and injuries had incapacitated about 3,500 of his soldiers. An additional 3,700 volunteers departed for home because their terms of enlistment had expired. While Scott awaited reinforcements for his army, which by then numbered about 6,000 soldiers, he was also awaiting the results of another fruitless attempt to end the war through peace negotiations.
Fall of Mexico City
When diplomacy failed, Scott began his final advance on Mexico City. In mid-August, he scored major victories over the Mexican army at the Battle of Contreras (August 19–20, 1847) and the Battle of Churubusco (August 20, 1847). Following the American victory at the Battle of Molina del Rey on the outskirts of Mexico City, on September 8, Scott’s last remaining major obstacle was the Chapultepec Castle.
Situated on a high hill, the Chapultepec Castle provided protection to the Mexican capital from the west and served as a military academy. Garrisoned by fewer than 1,000 Mexican soldiers and about 200 student cadets commanded by General Nicolás Bravo, the castle blocked Scott’s access to the capital. On September 12, Scott ordered an artillery barrage against the fortress that lasted all day. At 8 a.m. on September 13, the Americans stormed the castle in three columns. By 9 a.m., General Bravo surrendered. Despite Bravo’s capitulation, six cadets refused to abandon their posts and fought to the death. Mexican later immortalized the cadets as Los Niños Héroes, the “Child Heroes.”
After the fall of Chapultepec Castle, the Americans moved on irrepressibly. At about 1 a.m. on September 14, Santa Anna ordered his troops to abandon the city. After brief negotiations with city officials, Scott led his victorious army into the Mexican capital at 7 a.m., September 14, 1847.
Mexican-American War Aftermath
Following the American occupation of Mexico City, Santa Anna resigned as President of Mexico but continued to lead the country’s army. After regrouping his scattered forces, he unsuccessfully waged guerrilla warfare on Scott’s forces, especially his supply depot at Puebla. On October 11, 1847, American reinforcements arrived at Puebla and broke a Mexican siege, ending major hostilities.
Over the course of the war, the United States suffered about 17,000 casualties (13,000 dead and 4,000 wounded). Of the deaths, roughly 1,800 military personnel died in combat. The others died from sickness and disease. Mexican casualties totaled about 25,000, including possibly 1,000 civilians.
Treaty of Guadalupe Hidalgo
For the rest of 1847, American forces occupied or controlled most of the country, while Mexican officials struggled to organize a new national government. As the Mexicans put their house in order, American diplomat Nicholas P. Trist began negotiations to end the war. Trist and his Mexican counterparts reached an accord in January 1848. On February 2, 1848, representatives from both sides signed the Treaty of Guadalupe Hidalgo.
The peace treaty established the Rio Grande River as the southernmost border between Mexico and the United States. It also required Mexico to cede California, Arizona, New Mexico, and portions of Utah, Nevada, and Colorado to the United States, in return for a payment of $15 million. The U.S. Senate ratified the peace treaty on March 10, 1848, and the Mexican Congress did likewise on May 25.
Effects of the Mexican-American War
Besides the vast extension of American territories in the West, the Mexican-American War created at least three other unintended consequences in U.S. history.
- Zachary Taylor’s successes in Northern Mexico buoyed his political ambitions, enhanced his popularity, and propelled him to victory in the 1848 U.S. presidential election. Ironically, Winfield Scott, who enjoyed more military success during the war, never achieved his ambition of becoming President of the United States, despite coveting the White House more than Taylor.
- On the home front, the Mexican-American War crystallized and polarized sectional differences over the extension of slavery in the United States. Beyond the nationalistic gratification engendered by the fulfillment of America’s so-called Manifest Destiny, many southerners viewed the war as an opportunity to maintain their influence in the U.S. Senate by creating more slave states from territories carved out of the west. Opposing them, a small, but vocal, group of mostly northerners saw the war as a southern attempt to extend the evils of America’s “peculiar institution.” Kentucky Congressman Henry Clay denounced the war as one of “unnecessary and offensive aggression.” Henry David Thoreau went to jail rather than pay a tax to support American aggression in Mexico. On December 22, 1847, first-term Congressman (and future U.S. president) Abraham Lincoln introduced a resolution challenging President Polk to “establish whether the particular spot on which the blood of our citizens was so shed” (between the Rio Grande and Nueces rivers) “was or was not at that time our own soil.” Another U.S. president, Ulysses S. Grant (who fought in Mexico and later rose to the position of General-of-the-Army during the American Civil War) characterized the war as “as one of the most unjust ever waged by a stronger against a weaker nation.”
- Grant was not the only future Civil War general officer who served in Mexico. Although unforeseen a final unintended consequence of the Mexican-American War was its unfolding as a training ground for many general officers who served on both sides during the American Civil War. Among the more prominent of the future Confederate officers were Robert E. Lee, Joseph E. Johnston, Thomas J. Jackson, P. G. T. Beauregard, James Longstreet, A. P. Hill, Braxton Bragg, Richard Ewell, Earl Van Dorn, George Pickett, Gideon Pillow, Henry Heth, John B. Magruder, John C. Pemberton, Lafayette McLaws, Richard H. Anderson, Simon B. Buckner, Sterling Price, William J. Hardee, William Steele, and Confederate President Jefferson Davis. On the Union side, Winfield Scott, George B. McClellan, Henry Halleck, George G. Meade, William T. Sherman, Irvin McDowell, Ambrose Burnside, Don Carlos Buell, George Thomas, John Pope, Joseph Hooker, Winfield Scott Hancock, Abner Doubleday, Alfred Pleasonton, Charles F. Smith, Darius N. Couch, David Hunter, Edward Canby, Edward O. C. Ord, Edwin V. Sumner, Fitz John Porter, Frederick Steele, George Sykes, Gordon Granger, Henry H. Sibley, John C. Frémont, John F. Reynolds, John G. Foster, John Sedgwick, Nathaniel Lyon, Philip Kearny, Philip St. George Cooke, Samuel P. Heintzelman, Thomas J. Wood, Thomas L. Crittenden, William B. Franklin, and William S. Harney earned their mettle in Mexico. Serving together as they did, many of these men developed familiarities with each other that enabled them to anticipate each other’s actions once they became foes, thus shaping the course of the Civil War.
Mexican-American War APUSH Review, Notes, and Study Guide
Use the following links and videos to study the Mexican-American War, the Texas Revolution, and the Annexation of Texas for the AP US History Exam. Also, be sure to look at our Guide to the AP US History Exam.
Mexican-American War APUSH Definition
The Mexican-American War was fought by the United States and Mexico from 1846 and 1848, primarily over the Annexation of Texas and disputes over boundaries. It resulted in the United States acquiring significant portions of Mexican territory, including California and New Mexico, through the Treaty of Guadalupe Hidalgo in 1848, also known as the Mexican Cession. The war was a precursor to the Civil War and increased tension between pro-slavery and anti-slavery factions within the United States.
Mexican-American War Significance, Video for APUSH Notes
This video from Heimler’s History discusses the Mexican-American War and what students need to know for the APUSH exam.