The Mexican-American War

The Mexican-American War (or the U.S. – Mexico War) was the conflict that took place between 1846 and 1848 and caused Mexico to lose close to half of its territory and the U.S. to acquire what is today Texas, Arizona, New Mexico, Colorado, Nevada and California.

The Spanish in North America and the Treaty of San Lorenzo

Since the 16th century, the Spanish had been present in North America. Many of the Spanish-held territories eventually became part of the Viceroyalty of New Spain, which encompassed what is today Mexico. In the northeastern parts of the continent, France and England vied for control of the territory.

With the Independence of the 13 colonies from Great Britain, Spanish territory also found itself adjacent to the newly formed United States.

In order to define the border in North America, in 1795 Spain signed the Treaty of San Lorenzo with the U.S. Even so, these borders would shift over the next several years.

The Louisiana Purchase and the Adams-Onís Treaty

In 1803, the U.S. acquired the territory of Louisiana from France for $15 million. This deal, known as the Louisiana Purchase, gave the U.S. access to lands that included what is today the states of Arkansas, Oklahoma, Kansas, Missouri, Iowa, Nebraska, South Dakota and parts of Montana, Wyoming, Colorado, North Dakota, Minnesota, Texas, New Mexico and Louisiana, for a total of 828,000 square miles.

With this acquisition, the U.S. greatly increased the size of the land it claimed and multiplied the extent of its border with Spanish-claimed territories.

Almost two decades laters, in 1819, the U.S. and Spain signed the Adams-Onís Treaty. This agreement confirmed Spain’s claim to the territory of Texas, but allowed the U.S. to purchase what is today Florida.

The Colonization of Texas

Shortly before Mexican independence, in 1821 Spain granted a petition for the first group of Americans to settle in Texas. Until then, Americans were forbidden from occupying the Texas territory.

Mexico’s war of independence from Spain put the petition on hold, but after Mexico achieved independence, Stephen Austin — the son of the original petitioner — presented a new request to bring a group of 300 American settlers into Texas.

Since the Mexican government was having difficulty attracting settlers to the territory, it approved the petition. Mexico allowed Austin and his settlers to enter Texas on the condition that they be of good moral character, practice Catholicism and follow the laws of Mexico.

Although Austin’s permission was an unique, Mexico passed a colonization law in 1825 that allowed thousands of foreigners to settle in Texas. By 1835, there were 20,000 Americans (including Cherokee Indians) in Texas, with roughly 5,000 Mexicans.

Although Mexico had originally seen American immigrants as a way to shore up its control of the Texas territory, it eventually began to doubt their allegiance and view them with suspicion. Texans, in turn, became resentful at their small amount of representation in Mexican politics.

Texas Independence

The tensions in Texas reached a tipping point when General Antonio López de Santa Anna took office as president of Mexico in 1834. Soon after coming to power, he revoked the constitution, set about centralizing governmental authority in Mexico City, and reduced the autonomy of the individual states.

This caused a great deal of unrest in some of the territories. In September of 1835, Texans began a war of secession. In November, a group met in San Felipe de Austin and established a provisional state government .

After months of fighting with Santa Anna’s army, on March 2, 1836, Texas declared its independence from Mexico. For several weeks, Mexican army forces fought the insurgents, including at the famous battle of the Alamo, where Santa Anna’s troops defeated a group of Texas fighters holed up in the old mission. Despite this iconic loss, Sam Houston’s Texas army ultimately emerged victorious.

On April 21, Gen. Santa Anna, now captured, signed the Treaty of Velasco, recognizing Texas’ independence. Santa Anna’s capitulation was not the final word, though. The Mexican Congress refused to ratify the treaty since the general was a prisoner of war when he signed it. The United States, however, recognized Texas as an independent republic on March 3, 1837.

Although Mexico would not recognize an independent Texas, struggles within the country and lack of money prevented it from doing anything to retake it. In the meantime, the U.S. aspired to control the North American continent and to keep out rivals like France and Great Britain.

Prelude: The Short Seizure of Monterey

In 1842, the commander of the U.S. Pacific squadron, Commodore Thomas ap Catesby Jones, received news that French vessels had sailed from the coast of South America, possibly to California. He also learned that British ships might be en route to California. Armed with this news and a message from the U.S. consul at Mazatlán implying an imminent war with Mexico over Texas, Jones set sail for Alta California.

Although he had received no orders or official confirmation about a war, Jones believed it was his duty to strike.

On October 18, 1842, the commodore anchored his ships off the coast of Monterey, the capital of Mexican California. The next morning, he sent officers ashore to demand the surrender of Gov. Juan B. Alvarado and the military detachment of the Monterey Presidio. The presidio was defended by approximately 50 soldiers, many of which had no military instruction.

Jones had a force of 800 men with 80 cannons aboard his warships. With U.S. consul Thomas Larkin acting as an interpreter, Mexican officials soon signed the surrender documents.

Despite the surrender, Larkin insisted with the commodore that he had no news of war. Mexican documents seized during the occupation also showed no evidence of war. Eventually, Jones himself became convinced it was all a mistake.

On October 21, he ordered the U.S. flag lowered and the Mexican flag restored, apologizing for his actions. He also sent word to newly appointed Mexican governor Manuel Micheltorena, who had ordered the military commanders at Sonoma, Santa Barbara and Los Angeles to organize resistance against the Americans.

The Annexation of Texas

Although the incident at Monterey ended peacefully, events over the next few years pushed the two countries towards war. In 1843 Great Britain and France helped arrange a truce between Texas and Mexico.

This alarmed U.S. lawmakers, worried about their rivals’ interests in Mexico’s northwestern territories. In February of 1845, the U.S. Congress passed a joint resolution calling for annexing Texas.

In June of 1845, the Texas Convention of Representatives also approved the annexation, and U.S. President James K. Polk ordered U.S. troops into Texas. He also sent warships to the Gulf of Mexico and strengthened forces in the Pacific. In doing so, Polk supported Texas’ claims that its boundary with Mexico should be at the Río Grande (Río Bravo in Mexico), not further north at the Nueces River, as Mexico claimed.

President Polk also sent a representative to Mexico to negotiate a purchase California and New Mexico. Although the Mexican government was saddled with debts, it refused the offer.

In October of 1845, the people of Texas ratified the U.S. annexation, and by the end of December, Texas was part of the Union. Pres. Polk ordered U.S. troops to cross the Nueces River and deploy south.

The Start of the Mexican-American War

With Texas officially part of the U.S., tensions continue to rise. In April of 1846, the U.S. Navy blockaded the Río Grande, and Mexican General Pedro de Ampudia ordered the U.S. commander to withdraw his forces to north of the Nueces. When the U.S. refused, Mexico declared a defensive war on the United States, and Mexican forces attacked U.S. troops between the Nueces and the Río Grande. On May 13, 1846, the U.S. Congress declared war on Mexico.

With the U.S. now at war with Mexico over Texas, President Polk moved to resolve the long-simmering dispute with Great Britain over control of the Oregon territory along the northwestern coast of the Pacific.

In June of 1846, the U.S. and Great Britain signed an agreement, granting the U.S. control of everything below the 49th parallel. Once the Oregon issue was settled, the U.S. was free to focus on the war with Mexico.

The Capture of New Mexico and Monterrey

U.S. forces soon turned their attention on Mexico’s far northern territories. In August of 1846, U.S. forces, led by Col. Stephen Watts Kearney, captured Santa Fe after the Mexican governor of the territory, Manuel Armijo, dissolved his army and left for Chihuahua.

The next major strategic objective for the U.S. Army was the fortified city of Monterrey, the capital of northeastern Mexico and the Río Grande area. On September 25, after fierce fighting and artillery bombardments, the U.S. captured Monterrey. By the end of 1846, the U.S. controlled northeastern Mexico.

The Mexican-American War in California

Frémont’s Arrival

In January of 1846, U.S. Army Captain John C. Frémont arrived in the Salinas Valley on what was described as a scientific expedition. While there, he travelled to Monterey, to meet with U.S. Consul and confidential agent Thomas O. Larkin. There, he and Larkin and discussed the hostilities over Texas and U.S. plans for Alta California.

Mexican military commander José Castro, a Californio suspicious of the presence of a U.S. Army officer in California, soon ordered Frémont to leave.

Rather than follow Castro’s orders, the colonel and his men set up a fortified encampment on nearby Gavilan Peak, raising the American flag for all to see. Frémont remained in the area until March, when he moved north to Oregon. On May 8-9, he met up with Marine Lieutenant Archibald Gillespie, who had also entered California clandestinely and seeking to catch up to Frémont.

The Bear Flag Revolt

Frémont returned to Alta California in late May of 1846, accompanied by a detachment of Delaware Indians. He soon lent his support to a group of American immigrants in and around the town of Sonoma who were plotting a rebellion against Mexico.

The uprising, which would come to be known as the Bear Flag Revolt, took place on June 14. With Frémont’s support, the Osos (bears) as the Mexicans called the them, raised their homemade grizzly bear flag over the town of Sonoma and took military commander Gen. Mariano Guadalupe Vallejo prisoner. Their flag would eventually become the basis for the California state flag adopted in 1911.

San Francisco and Monterey

On July 7, naval Commodore John Drake Sloat arrived off the coast of Monterey. He ordered the Marines ashore to raise the American flag, and five days later did the same thing at San Francisco. In the meantime, Frémont incorporated the Osos into the Army’s newly-formed California Battalion and made his way to Monterey.

Along the way, their violence enraged both Americans and Californios, who accused them of wanton murder. Accompanied by mountain man and army scout Kit Carson, Frémont’s troops boarded ships bound for San Diego as reinforcements for the campaign in southern California.

Battles in Southern California

By early August, U.S. troops had control of the Pueblo of Los Angeles, and Mexican Gov. Pío Pico had fled south to Baja California, vowing to return. The city chafed under occupation, and an uprising eventually forced Marine Capt. Gillespie, the U.S. commander, to flee the city. On December 6, a force of Californios defeated U.S. troops under (now General) Kearney, at the Battle of San Pascual in today’s San Diego County.

The Treaty of Cahuenga

In January, the U.S. mounted a full-blown invasion of Los Angeles, with soldiers, marines and sailors, as well as a unit of artillery. The invasion was successful in overwhelming the Californios, and on January 10, 1847, Los Angeles again fell to the U.S. at the Battle of La Mesa.

The Californios surrendered three days later, at Campo de Cahuenga (today’s Universal City). The agreement that ended the fighting came to be known as the Treaty of Cahuenga.

The March Toward Mexico City

With Mexico’s northwestern territories in his control, President Polk decided to take aim at the capital of Mexico, Mexico City. Retracing the route traveled by Hernán Cortez in 1519, U.S. troops landed at Veracruz on March 9, 1847. Veracruz fell a on March 29, following days of bombardment.

On May 15, 4,000 U.S. troops captured the city of Puebla, after defeating Mexican forces at the battle of Cerro Gordo on April 18.

With the fall of Puebla, the U.S. Army began its march toward Mexico City, battling Santa Anna’s troops along the way. After Santa Anna rejected a truce offered by American Major General Winfield Scott, U.S. forces took up positions outside of Mexico City on September 13. They soon began bombarding historic Chapultapec Castle, located on a hill overlooking the city.

The castle housed a military academy, and the soldiers and cadets within the castle refused to surrender. The next morning, U.S. soldiers stormed and took the castle.

U.S. forces next moved to the gates of the city, where fighting with Mexican troops broke out. The battle spread into the city, and lasted until nightfall. But after civilian officials in Mexico City convinced Santa Anna to withdraw his troops to avoid more bloodshed, American forces took control of the capital. The Mexican government relocated its headquarters to Querétaro, approximately 135 miles away.

U.S. Victory and the End of the Mexican-American War

On September 16, 1847, Gen. Antonio López de Santa Anna announced his resignation, and soon left the country for exile in Kingston, Jamaica. The United States Army went from being an invasion force to an army of occupation. Troops remained in Mexico City from September 14, 1847 to June 12, 1848.

With the effective capitulation of Mexican forces, the two governments had began negotiating a treaty. The opinion of legislators in the U.S., however, was divided regarding how to proceed regarding Mexico. In the Congress there even arose a movement to annex the entire country, which would come to be known as the “All Mexico” movement.

The Treaty of Guadalupe Hidalgo

Nevertheless, in less than a month, a treaty between the two countries on February 2 at Villa de Guadalupe Hidalgo (now a neighborhood of Mexico City). The agreement came to be known as the Treaty of Guadalupe Hidalgo.
In it, the U.S. committed to compensate Mexico $15 million, and pay off the claims of U.S. citizens against Mexico for up to $5 million.

The treaty solidified the Texas border at the Río Grande and conceded Alta California (which included much of today’s Arizona, as well as Nevada, Utah and parts of Colorado) and New Mexico. Mexican citizens had the right to remain where they were, or to go to Mexico. Those who chose to remain could continue as Mexican citizens or receive American citizenship. They were to retain all of their rights as well as their property. The U.S. Senate ratified the treaty on March 10, 1848.

The Gadsden Purchase

Although a peace treaty had been signed, over the next several years, tensions simmered between the U.S. and Mexico over disputed territories in Mexico’s Mesilla Valley, which would later become parts of Arizona and New Mexico.

In December of 1853, the two countries signed a deal that ceded the territory to the U.S. for $10 million. The agreement, known as the Gadsden Purchase, was ratified in 1854, and granted the U.S. the land it needed to finish building the Transcontinental Railroad.

Frequently Asked Questions

How did Mexico lose its land to the U.S.?

Mexico lost its land to the U.S. as a result of the U.S. annexation of Texas and the Mexican-American War.

What was the US Mexican War?

The Mexican-American War (or the U.S. – Mexico War) was the conflict that took place between 1846 and 1848 and caused Mexico to lose close to half of its territory and the U.S. to acquire what is today Texas, Arizona, New Mexico, Colorado, Nevada and California.

What was the main cause of the Mexican American War?

The main cause of the Mexican-American War were disputes over lands in Mexico’s northern territories, especially the U.S. annexation of Texas.

When did Mexico and America go to war?

Mexico and the U.S. went to war in 1846.

How did the U.S. get Texas?

The U.S. acquired Texas through annexation, with the people of Texas voting to ratify the annexation. U.S. control of Texas was later solidified through the Mexican-American War.

Who started the Mexican-American War and why?

In April of 1846, after the U.S. annexation of Texas, the U.S. Navy blockaded the Río Grande. When the U.S. refused to move back to positions north of the Nueces River, Mexico declared a defensive war on the United States, and Mexican forces attacked U.S. troops between the Nueces and the Río Grande. On May 13, 1846, the U.S. Congress declared war on Mexico.

Who was the U.S. president during the Mexican American War?

James K. Polk was the U.S. president during the Mexican-American War.

Did the U.S. take land from Mexico?

The U.S. seized land from Mexico during military campaigns as part of the war between the two countries.

Who won the Mexican American war?

The U.S. won the Mexican-American War.

Why did the U.S. pay Mexico 15 million dollars?

The U.S. paid Mexico 15 million dollars for territories it acquired as the result of the Mexican-American War.

Did Mexico sell land to the U.S.?

Mexico sold land to the U.S. in 1853 as part of the Gadsden Purchase.

For More Information

The Dead March: A History of the Mexican-American War by Peter Guardino. One of the most thorough recent books on the Mexican-American War, and a winner of several national book awards.

California Conquered: : The Annexation of a Mexican Province, 1846-1850 by Neal Harlow. An in-depth and engaging account of the Mexican-American War in California.

The U.S. War with Mexico: A Brief History with Documents by Ernesto Chávez. A very concise overview of the Mexican-American War with the full texts of all important documents related to the war.