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Introduction
The Mexican-American War, or Mexican War, was fought between Mexico and the United States from 1846 to 1848. "Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon American soil. War exists, and, notwithstanding all our efforts to avoid it, exists by the act of Mexico herself." So U.S. President James K. Polk said in his message to Congress in May 1846.

The trouble grew out of a dispute over the border between the two countries. Texas, formerly a part of Mexico, claimed that its southwest boundary extended to the Rio Grande. When Texas entered the Union and became a U.S. state, this was the boundary accepted by the United States. Mexico claimed that the boundary was the Nueces River, which is 100 miles (160 kilometers) eastward.

In August 1845, President Polk stationed American troops west of the Nueces, showing that the United States would protect Texas to the Rio Grande but attempting at the same time to avoid war. But on January 13, 1846, he ordered General Zachary Taylor to advance to the Rio Grande. This was a frank invasion of Mexican territory. A Mexican force crossed the river at Palo Alto, and a battle took place on May 8 of that year. This was followed on May 9 by the Battle of Resaca de la Palma. The United States declared war on Mexico on May 13.

California and the Slavery Question
Two other issues complicated the matter. In the first place many American citizens had claims against Mexico for losses of property caused by the unsettled condition of the country. Mexico agreed to pay only part of the claims. The U.S. president also wished to acquire the vast region of California during his administration. He had tried in vain to purchase California, but Mexico had refused to sell its territory. The refusal had added fuel to the flame, especially for those Americans who wanted more land for the expansion of slavery. Antislavery New England opposed the war.

A One-Sided Conflict
In almost every battle in the war the American troops were victorious, though they were outnumbered by the Mexicans by about two to one. After taking Resaca de la Palma, General Taylor struck into the interior of Mexico. After a four-day siege and a gallant resistance by the Mexicans, he entered the city of Monterrey on September 24. His most famous victory was that of Buena Vista. There, for the entire day of February 23, 1847, his army of 4,700 men successfully withstood the attack of 20,000 Mexicans under Antonio López de Santa Anna. The Mexicans finally acknowledged defeat by withdrawing from the field.

While these victories were being won in Mexico, Americans were taking possession of California. U.S. Navy Commodore John D. Sloat was stationed in Mazatlán, Mexico, at the outbreak of the war. He sailed to Monterey, California, and took possession of it on July 7, 1846. Yerba Buena (now San Francisco), California, was seized the next day. In August 1846, Sloat relinquished command of the Pacific squadron to Commodore Robert F. Stockton. With Captain John C. Frémont he completed the conquest of California. In January 1847 the last areas of resistance were brought under control by General Stephen W. Kearny, who had earlier secured New Mexico.

On March 27, 1847, General Winfield Scott captured the fortress on the harbor of Veracruz. This fort controlled the gateway to Mexico. On April 18, with 8,000 men, he drove some 12,000 Mexicans from the pass of Cerro Gordo. Finally, he came to the Rock of Chapultepec (Grasshopper Hill), 3 miles (4.8 kilometers) from Mexico City. The Mexicans fought heroically in defense of this ancient fortress; however, after a desperate struggle they were defeated. On September 14, American troops entered Mexico City, and the fighting was at an end.

The Treaty of Peace
By the Treaty of Guadalupe-Hidalgo (February 2, 1848), Mexico accepted the Rio Grande as its boundary. The treaty also gave the United States Mexico's northern provinces of California and New Mexico. These included parts of the present states of Nevada, Utah, Arizona, Colorado, and Wyoming. Mexico received 15 million dollars and was also relieved of all claims of citizens of the United States against it.

The United States thus acquired vast mineral wealth, especially gold, and a door to the Pacific Ocean and to East Asia. The additional territory raised again the burning question of slavery in the U.S. and the application of the Missouri Compromise to the new lands. The questionable character of the American claims to Mexican lands contributed a heritage of ill will to the troubled relations between the United States and its southern neighbor, Mexico.

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Introduction The Mexican-American War, or Mexican War, was fought between Mexico and the United States from 1846 to 1848. "Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon American soil. War exists, and, notwithstanding all our efforts to avoid it, exists by the act of Mexico herself."

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Table of Contents

1. Causes of the Mexican-American War

2. The Mexican-American War Begins

3. U.S. Army Advances Into Mexico

4. Treaty of Guadalupe Hidalgo&nbsp;

The Mexican-American War of 1846 to 1848 marked the first U.S. armed conflict chiefly fought on foreign soil. It pitted a politically divided and militarily unprepared Mexico against the expansionist-minded administration of U.S. President James K. Polk, who believed the United States had a "Manifest Destiny" to spread across the continent to the Pacific Ocean. A border skirmish along the Rio Grande that started off the fighting was followed by a series of U.S. victories. When the dust cleared, Mexico had lost about one-third of its territory, including nearly all of present-day California, Utah, Nevada, Arizona and New Mexico.

Causes of the Mexican-American War

Texas gained its independence from Mexico in 1836. Initially, the United States declined to incorporate it into the union, largely because northern political interests were against the addition of a new state that supported slavery. The Mexican government was also encouraging border raids and warning that any attempt at annexation would lead to war.

Did you know? Gold was discovered in California just days before Mexico ceded the land to the United States in the Treaty of Guadalupe Hidalgo.

Nonetheless, annexation procedures were quickly initiated after the 1844 election of Polk, a firm believer in the doctrine of Manifest Destiny, who campaigned that Texas should be "re-annexed" and that the Oregon Territory should be "re-occupied." Polk also had his eyes on California, New Mexico and the rest of what is today the American Southwest.

When his offer to purchase those lands was rejected, he instigated a fight by moving troops into a disputed zone between the Rio Grande and Nueces River that both countries had previously recognized as part of the Mexican state of Coahuila.

The Mexican-American War Begins

On April 25, 1846, Mexican cavalry attacked a group of U.S. soldiers in the disputed zone under the command of General Zachary Taylor, killing about a dozen. They then laid siege to Fort Texas along the Rio Grande. Taylor called in reinforcements, and-with the help of superior rifles and artillery-was able to defeat the Mexicans at the Battle of Palo Alto and the Battle of Resaca de la Palma.

Following those battles, Polk told the U.S. Congress that the "cup of forbearance has been exhausted, even before Mexico passed the boundary of the United States, invaded our territory, and shed American blood upon American soil." Two days later, on May 13, Congress declared war, despite opposition from some northern lawmakers. No official declaration of war ever came from Mexico.

U.S. Army Advances Into Mexico

At that time, only about 75,000 Mexican citizens lived north of the Rio Grande. As a result, U.S. forces led by Col. Stephen Watts Kearny and Commodore Robert Field Stockton were able to conquer those lands with minimal resistance. Taylor likewise had little trouble advancing, and he captured the city of Monterrey in September.

With the losses adding up, Mexico turned to old standby General Antonio López de Santa Anna, the charismatic strongman who had been living in exile in Cuba. Santa Anna convinced Polk that, if allowed to return to Mexico, he would end the war on terms favorable to the United States.

But when Santa Anna arrived, he immediately double-crossed Polk by taking control of the Mexican army and leading it into battle. At the Battle of Buena Vista in February 1847, Santa Anna suffered heavy casualties and was forced to withdraw. Despite the loss, he assumed the Mexican presidency the following month.

Meanwhile, U.S. troops led by Gen. Winfield Scott landed in Veracruz and took over the city. They then began marching toward Mexico City, essentially following the same route that Hernán Cortés followed when he invaded the Aztec empire.

The Mexicans resisted at the Battle of Cerro Gordo and elsewhere, but were bested each time. In September 1847, Scott successfully laid siege to Mexico City's Chapultepec Castle. During that clash, a group of military school cadets-the so-called ni ños héroes -purportedly committed suicide rather than surrender.

Treaty of Guadalupe Hidalgo

Guerrilla attacks against U.S. supply lines continued, but for all intents and purposes the war had ended. Santa Anna resigned, and the United States waited for a new government capable of negotiations to form.

Finally, on Feb. 2, 1848, the Treaty of Guadalupe Hidalgo was signed, establishing the Rio Grande (and not the Nueces River) as the U.S.-Mexican border. Under the treaty, Mexico also recognized the U.S. annexation of Texas, and agreed to sell California and the rest of its territory north of the Rio Grande for $15 million plus the assumption of certain damage claims.

The net gain in U.S. territory after the Mexican-American War was roughly 525,000 square miles, an enormous tract of land-nearly as much as the Louisiana Purchase's 827,000 square miles-that would forever change the geography, culture and economy of the United States.

Though the war with Mexico was over, the battle over the newly acquired territories-and whether or not slavery would be allowed in those territories-was just beginning. Many of the U.S. officers and soldiers in the Mexican-American War would in just a few years find themselves once again taking up arms, but this time against their own countrymen in the Civil War.

HISTORY Vault

Sources

The Mexican American War. PBS: American Experience.
The Mexican-American war in a nutshell. Constitution Daily.
The Mexican-American War. Northern Illinois University Digital Library..

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Mexican-American War: Causes & Treaty of Guadalupe Hidalgo | HISTORY

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The Mexican-American War was a 1846-1848 conflict over vast territories in the American West, which the Treaty of Guadalupe Hidalgo gave to the United States.

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Use for more information if necessary. Feel free to use this as a source of evidence for #7.

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Ulysses S. Grant | Article

Print shows a scene from the Battle of Monterrey, led by General Zachary Taylor and General William J. Worth, during the Mexican-American War.1855. Library of Congress.

In 1844, President James K. Polk ran on a Democratic platform that supported manifest destiny, the idea that Americans were predestined to occupy the entire North American continent. The last act of Polk's predecessor, John Tyler, had been to annex the Republic of Texas in 1845. Polk wanted to lay claim to California, New Mexico, and land near the disputed southern border of Texas. Mexico, however, was not so eager to let go of these territories.

Polk started out by trying to buy the land. He sent an American diplomat, John Slidell, to Mexico City to offer $30 million for it. But the Mexican government refused to even meet with Slidell. Polk grew frustrated. Determined to acquire the land, he sent American troops to Texas in January of 1846 to provoke the Mexicans into war.

When the Mexicans fired on American troops in April 25, 1846, Polk had the excuse he needed. He declared, "[Mexico] has invaded our territory and shed American blood upon American soil," and sent the order for war to Congress on May 11.

The act was a questionable one. Many Northerners believed that Polk, a Southerner, was trying to gain land for the slaveholding South. Other Americans simply thought it was wrong to use war to take land from Mexico. Among those was Second Lieutenant Ulysses S. Grant. Although during the war he expressed no reservations about it, he would later call the war "one of the most unjust ever waged by a stronger against a weaker nation. It was an instance of a republic following the bad example of European monarchies, in not considering justice in their desire to acquire additional territory."

Despite arguments over whether the war was right, Americans had tremendous success on the battlefield. Young officers like Grant and Robert E. Lee, who would later lead armies against one another in the Civil War, had their first combat experiences in Mexico. Generals Zachary Taylor and Winfield Scott won a series of remarkable victories against the Mexican armies. This success was in spite of the fact that Mexican troops outnumbered the Americans in most cases. In September of 1847, after a masterful overland campaign, American troops under Scott captured Mexico's capital, Mexico City, and the fighting ended.

The Treaty of Guadalupe Hidalgo sealed the American victory in 1848. In return for $15 million and the assumption of Mexican debts to Americans, Mexico gave up its hold over New Mexico and California. The enormous territory included present-day California, Nevada, Utah, and parts of Arizona, New Mexico, Oklahoma, Colorado and Wyoming. Mexico also agreed to finally relinquish all of Texas, including the disputed area along the border. The U.S. Congress approved the treaty on March 10.

Although the Mexican War had been won, the conflict over what to do with the vast amounts of territory gained from the war sparked further controversy in the U.S. The question over whether slavery would spread to these new territories would drive North and South even further apart.

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The Mexican American War | American Experience | PBS

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Determined to acquire the land, Polk sent American troops to Texas in January of 1846 to provoke the Mexicans into war.

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Available if you'd like a deeper look at the Treaty of Guadalupe Hidalgo. Feel free to use this as a source of evidence for #7.

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En Español
The Treaty of Guadalupe Hidalgo, that brought an official end to the Mexican-American War (1846-1848), was signed on February 2, 1848, at Guadalupe Hidalgo, a city north of the capital where the Mexican government had fled with the advance of U.S. forces. By its terms, Mexico ceded 55 percent of its territory, including parts of present-day Arizona, California, New Mexico, Texas, Colorado, Nevada, and Utah, to the United States. Mexico relinquished all claims to Texas, and recognized the Rio Grande as the southern boundary with the United States. Read more...

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Additional Background Information

With the defeat of its army and the fall of the capital, Mexico City, in September 1847 the Mexican government surrendered to the United States and entered into negotiations to end the war. The peace talks were negotiated by Nicholas Trist, chief clerk of the State Department, who had accompanied General Winfield Scott as a diplomat and President Polk's representative. Trist and General Scott, after two previous unsuccessful attempts to negotiate a treaty with Santa Anna, determined that the only way to deal with Mexico was as a conquered enemy. Nicholas Trist negotiated with a special commission representing the collapsed government led by Don Bernardo Couto, Don Miguel Atristain, and Don Luis Gonzaga Cuevas of Mexico.

President Polk had recalled Trist under the belief that negotiations would be carried out with a Mexican delegation in Washington. In the six weeks it took to deliver Polk's message, Trist had received word that the Mexican government had named its special commission to negotiate. Against the president's recall, Trist determined that Washington did not understand the situation in Mexico and negotiated the peace treaty in defiance of the president. In a December 4, 1847, letter to his wife, he wrote, "Knowing it to be the very last chance and impressed with the dreadful consequences to our country which cannot fail to attend the loss of that chance, I decided today at noon to attempt to make a treaty; the decision is altogether my own."

Ignoring the president's recall command with the full knowledge that his defiance would cost him his career, Trist chose to adhere to his own principles and negotiate a treaty in violation of his instructions. His stand made him briefly a very controversial figure in the United States.

Under the terms of the treaty negotiated by Trist, Mexico ceded to the United States Upper California and New Mexico. This was known as the Mexican Cession and included present-day Arizona and New Mexico and parts of Utah, Nevada, and Colorado (see Article V of the treaty). Mexico relinquished all claims to Texas and recognized the Rio Grande as the southern boundary with the United States (see Article V).

The United States paid Mexico $15,000,000 "in consideration of the extension acquired by the boundaries of the United States" (see Article XII of the treaty) and agreed to pay American citizens debts owed to them by the Mexican government (see Article XV). Other provisions included protection of property and civil rights of Mexican nationals living within the new boundaries of the United States (see Articles VIII and IX), the promise of the United States to police its boundaries (see Article XI), and compulsory arbitration of future disputes between the two countries (see Article XXI).

Trist sent a copy to Washington by the fastest means available, forcing Polk to decide whether or not to repudiate the highly satisfactory handiwork of his discredited subordinate. Polk chose to forward the treaty to the Senate. When the Senate reluctantly ratified the treaty (by a vote of 34 to 14) on March 10, 1848, it deleted Article X guaranteeing the protection of Mexican land grants. Following the ratification, U.S. troops were removed from the Mexican capital.

To carry the treaty into effect, commissioner Colonel Jon Weller and surveyor Andrew Grey were appointed by the U.S. Government, and General Pedro Conde and Sr. Jose Illarregui were appointed by the Mexican government, to survey and set the boundary. A subsequent treaty, the Gadsen Purchase, of December 30, 1853, altered the border from the initial one by adding 47 more boundary markers to the original six. Of the 53 markers, the majority were rude piles of stones; a few were of durable character with proper inscriptions.

As time passed, it became difficult to determine the exact location of the markers, with both countries claiming the originals had been moved or destroyed. To solve the problem, a convention between the two countries was concluded in the 1880s; and a survey was done that verified the need for definite demarcation of the boundary. The International Boundary Commission was created to relocate the monuments and mark the boundary line. The U.S. commissioners employed a survey photographer to record various views of each monument located and erected by the U.S. Section.

This text was adapted from an article written by Tom Gray, a teacher at DeRuyter Central Middle School in DeRuyter, NY.
Materials created by the National Archives and Records Administration are in the public domain.

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En Español The Treaty of Guadalupe Hidalgo, that brought an official end to the Mexican-American War (1846-1848), was signed on February 2, 1848, at Guadalupe Hidalgo, a city north of the capital where the Mexican government had fled with the advance of U.S. forces.