Mexican American War The Mexican War between the United States and Mexico

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Mexican American War

The Mexican War between the United States and Mexico began with a Mexican attack on American troops along the southern border of Texas on Apr. 25, 1846. Fighting ended when U.S. Gen. Winfield Scott occupied Mexico City on Sept. 14, 1847; a few months later a peace treaty was signed. In addition to recognizing the U.S. annexation of Texas defeated Mexico was forced to also give California and New Mexico (including all the present-day states of the Southwest) to the United States.

BACKGROUND - Causes of the War

As with all major events, historical interpretations concerning the causes of the Mexican War vary. Some people claim that the dictator of Mexico began the war because the U.S. annexed Texas (1845), which Mexico continued to claim despite the establishment of the independent republic of Texas 10 years before. Others have argued that the United States provoked the war by annexing Texas and, more deliberately, by stationing an army at the mouth of the Rio Grande. Another, related, interpretation maintains that U.S. President James Polk forced Mexico to war in order to seize California and the Southwest. A small minority of people believe that the war began because of Mexico's failure to pay claims for losses sustained by U.S. citizens during the Mexican War of Independence.

The U.S. annexation of Texas had caused considerable political debate in the United States. The desire of the Texas Republic to join the United States had been blocked for several years by antislavery forces, which feared that several new slave states would be created from the Texas territory. The principal factor that led the US Government to take action was British interest in independent Texas. Indeed, anti-British feeling lay behind most of the expansionist policy statements of the United States in this period. James Polk won the 1844 presidential election by being in favor of taking a hard stand against Britain on the Oregon Question. Once in office he declared that "the people of this continent alone have the right to decide their own destiny." About the same time the term Manifest Destiny came into vogue to describe what was regarded as a God-given right to expand U.S. territory. American settlers also warned of British interests to take control of California and Texas.

The Mexican Response and the Slidell Mission

As early as August 1843, Santa Anna's government had informed the United States that it would "consider equivalent to a declaration of war . . . the passage of an act for the incorporation of Texas." The Mexican government led by Herrera did not support this militant position. It had already started steps, encouraged by the British, to recognize the independence of the Republic of Texas, and although Santa Anna's minister in Washington broke diplomatic relations with the U.S. government immediately after annexation, in August 1845 the Herrera government indicated willingness to resume relations. Not only was the Herrera government prepared to accept the loss of Texas, but it also hoped to put to rest the claims question that had plagued U.S.-Mexican affairs since 1825. Britain and France had used force, or the threat of it, to “convince” the Mexican government to pay their claims on behalf of their citizens. The United States, however, preferred to negotiate, and the negotiations had dragged on.

Fearing that American patience was running short, Herrera seemed determined to settle the issue. He requested that the United States send a minister to Mexico, and President Polk appointed John Slidell.

Slidell's authority, however, may have exceeded Herrera's intentions. Slidell was authorized to purchase California and New Mexico from Mexico and to settle the Texas boundary, which was a source of dispute even with the Mexican moderates. While the Republic of Texas had claimed the Rio Grande as its boundary, the neighboring Mexican state of Tamaulipas claimed the area north of the Rio Grande to the Nueces River.

By the time Slidell arrived in Mexico in December 1845, the Herrera government was under intense fire from the Centralists for its moderate foreign policies. The Centralist strategy was to appeal to Mexican national pride as a means of ousting Herrera. During August 1845 the Centralist leader began to demand an attack on the United States. When Slidell arrived, Herrera, in an effort to save his government, refused to meet with him. A few days later, Mexican General Parades overthrew the Mexican government and ordered Slidell out of Mexico.

After the failure of the Slidell mission, President Polk ordered Zachary Taylor to move his army to the mouth of the Rio Grande and to prepare to defend Texas from invasion. Taylor did so, arriving at the Rio Grande on Mar. 28, 1846. Abolitionists in the United States, who had opposed the annexation of Texas as a slave state, claimed that the move to the Rio Grande was a hostile and aggressive act by Polk to provoke a war with Mexico to add new slave territory to the United States.

Whatever Polk's precise intentions were, in Mexico the annexation of Texas had been sufficient cause for war; they saw no disputed boundary--Mexico owned all of Texas. Before Taylor had moved to the Rio Grande, Parades had begun mobilizing troops and had stated again his intention of attacking. On April 4 the new dictator of Mexico ordered the attack on General Taylor. When the local commander delayed attacking the American forces, Parades replaced him, issued a declaration of war, and reordered the attack.


On Apr. 25, 1846, Mexican troops crossed the Rio Grande and ambushed a detachment of American dragoons commanded by Capt. John Hardee. Taylor's report of this ambush reached President Polk on the evening of May 9, a Saturday. On Monday, May 11, Polk presented his war message to Congress, and on Wednesday, May 13, over the vigorous opposition of the abolitionists, the U.S. Congress voted to declare war on Mexico. In the meantime two more Mexican attacks had been made across the Rio Grande, and both had been repulsed.

Mexican Expectations

Mexican leaders clearly expected to win these battles as well as to recover Texas and win the war. Mexican President Parades spoke grandly of occupying New Orleans and Mobile. His army of about 32,000 men was four to six times the size of the original U.S. army. Furthermore, Mexican troops were well armed, disciplined, and, above all, experienced in scores of revolutions. Parades also counted on logistics. The principal theater of war would be Texas, hundreds of miles from the populated areas of the United States. Many Mexicans believed that abolitionists' (an abolitionist was someone who wanted to abolish, or end, slavery) objections to the war would demoralize the United States, and some Mexicans believed that a Mexican invasion would be supported by a massive slave uprising.

Thus, the quick Mexican defeats surprised and shocked their leaders. The U.S. victories against a larger, better trained force were attributed to the unexpected effectiveness of the American light artillery. Parades found it easier to blame the on his commanding general, and he quickly replaced him. The Mexican garrison evacuated Matamoros, moving to the south.

Taylor's Delay

Taylor occupied Matamoros but then delayed for several months before moving south. He was apparently waiting for wagons that were promised him by the U.S. government, though his critics branded him incompetent. In July he moved his base up the Rio Grande, and the following month Taylor began planning the attack on Monterrey.

By that time American strength on the Rio Grande had swollen to nearly 20,000 troops, nearly all volunteers. The principal military problem was logistical support of such a quickly expanded force. The Americans were susceptible to subtropical diseases and found it difficult to maintain sanitary conditions in the camps. Fevers, dysentery, and general debility were rampant, and the mortality rate from sickness was alarming. A determined Mexican attack in July or August would have proven disastrous to the Americans.

Mexican Rebellion

The Mexicans did not attack because the Centralist government was collapsing. Rather than uniting Mexico, the war had given the Federalist faction an opportunity to rebel. Even while Taylor had been camped on the Nueces in the fall of 1845, a few Federalist leaders had been in contact with him, promising supplies and asking for assistance in overthrowing Parades. Northern Mexico was almost a Federalist stronghold, and as Taylor moved to the Rio Grande, he received increasing support from the rebels.

The defeats of the Centralist forces at Palo Alto and Resaca de la Palma resulted in open Federalist rebellions throughout Mexico. Major outbreaks at Acapulco and Guadalajara in July were followed by the defection of the military garrison of Veracruz and that of Mexico City. Mexico was in turmoil.

In July Parades turned the government over to his vice-president and went into hiding. The Centralists' government fell completely with the resignation of the vice-president a couple of days later. Then the Federalists restored the constitution and a new government came to power.

In the meantime, Santa Anna had returned to Mexico. Having promised President Polk that he would work to effect a truce, he was allowed to pass through the U.S. naval blockade and land at Veracruz. Talk of a truce was forgotten. Perhaps the only leader capable of uniting the nation, he soon received command of the Mexican army; he was then elected president by the Mexican Congress.

Monterrey and Buena Vista

In the meantime, Taylor began his advance on Monterrey. He reached that fortified town, which had a garrison of more than 10,000 troops, and began his attack on the morning of September 21. With about 2,000 men the Americans captured a major road into the city and by noon were storming Federation Hill. Six companies of Texas Rangers charged up the hill, seized the enemy artillery, and turned the cannon on retreating Mexican forces. After bloody street-to-street fighting, the Mexicans surrendered and were allowed to leave the city.

In January 1847, Santa Anna moved north with about 20,000 men to dislodge Taylor. Dispatches captured by the Mexicans had revealed that most of Taylor's forces were being withdrawn to take part in Gen. Winfield Scott's proposed landing at Veracruz. Word of Santa Anna's approach reached Taylor, and although outnumbered almost three-to-one, he took up a position at the town of Buena Vista. The Mexican attacked and forced the Americans to abandon important defensive positions. The next morning the main Mexican force nearly overcame the U.S. defense. However, a dramatic charge led by Col. Jefferson Davis (a man eventually elected as president of the Confederate States of America during the American Civil War) and a determined artillery advance under Capt. Braxton Bragg (a future Confederate General during the Civil War) finally saved the day for the Americans.


The decisive campaign of the war was Scott's advance from Veracruz to Mexico City. Scott's expedition began at a staging area at the mouth of the Rio Grande in February 1847. He assembled an army of approximately 12,000, which was transported by sea to a beach about 3 miles south of Veracruz. Landing on March 10-11, it had surrounded the city by March 15. A combined naval and land attack began a couple of days later and heavy shelling from navy guns forced the almost impregnable town to surrender a week later.

Cerro Gordo and Puebla

Almost immediately Scott began the advance toward Mexico City. Only sporadic resistance was encountered until his army reached the village of Cerro Gordo about 50 miles inland. There, in a narrow valley, Santa Anna prepared to turn back the Americans. The attack on Cerro Gordo was led by units under William J. Worth on April 18. The U.S. engineers, who included Robert E. Lee (future commander of Confederate forces), George B. McClellan (future commander of Union forces), Joseph E. Johnston (a future leading general of Confederate forces), and P. G. T. Beauregard (another future Confederate general), found a trail that enabled the Americans to surround and defeat Santa Anna's forces.

Contreras, Churubusco, and Chapultepec

During June and July, Santa Anna frantically prepared to defend Mexico City. On August 7, Scott began his advance from Puebla, following a route over lava beds and rough land to the south of Lake Chalco that Santa Anna had left relatively unprotected. The heavy fighting occurred outside Mexico City and the Mexicans were defeated, although Santa Anna and much of his command escaped.

After several battles American troops entered Mexico City and the war was over. In just over five months, Winfield Scott had done what many had considered impossible. The duke of Wellington wrote, "His campaign was unsurpassed in military annals." Santa Anna resigned the Mexican presidency and fled the country.


While the crucial fighting was taking place in Mexico, various U.S. expeditions affected the conquest of Mexico's territories in the American Southwest.

Kearny in New Mexico

Immediately after the declaration of war, Brigadier General Stephen Watts Kearny was ordered to occupy New Mexico and California. With an army consisting largely of Missouri volunteers and numbering fewer than 2,000 he moved down the Santa Fe Trail into New Mexico. The Mexican governor was unable to rally any resistance, and Kearny entered Santa Fe unopposed. The conquest of New Mexico had, in fact, taken place through peaceful trade and commerce in the preceding years.

Kearny established a civil government with Charles Bent, a Santa Fe trader from Missouri, as governor. He then divided his command: one was to occupy New Mexico, and another headed for California. The group in New Mexico faced unrest and rebellion mainly from the Pueblo Indians, whom were eventually defeated. Bent was murdered at his home in Taos.

Kearny set out for California on September 25 with only 300 dragoons. In New Mexico they met the famous guide Kit Carson, who was returning from California. Learning that the conquest of California was virtually complete, Kearny sent 200 of his men back to Santa Fe and, led by Carson, continued to California.

Conquest of California

The American settlers in California had revolted against Mexican rule and established (June 1846) the Bear Flag Republic, under John C. Fremont, before news of the war reached them. On July 2, U.S. troops landed at Monterey. They proclaimed U.S. jurisdiction on July 7 and two days later occupied San Francisco. However, California was by no means under U.S. control. Mexican authority in California was divided between two rivals, one in Los Angeles and the other in Monterey. Following the American landing, Castro headed south, apparently to attempt reconciliation with Pico and resistance to the United States. However, Commodore Robert Stockton sailed down the coast and landed troops under Fremont at San Diego and others near Los Angeles. Pico and Castro fled.

Heavy-handed martial law administration by the Americans caused a revolt in southern California in September. The rebels had expelled the Americans from Los Angeles and San Diego by the end of October. On Dec. 6, 1846, Kearny, en route to San Diego, met the rebels in a battle with no clear winner. In January Fremont received the final surrender of the rebels and signed a treaty securing control of California.


Despite the objections of the abolitionists, the war received enthusiastic support in all sections of the United States and was fought almost entirely by volunteers. The army swelled from just over 6,000 to over 115,000. Of this total approximately 1.5 percent were killed in the fighting, and nearly 10 percent died of disease; another 12 percent were wounded or discharged because of disease or both. For years afterward, Mexican War veterans continued to suffer from the debilitating diseases contracted during the campaigns. The casualty rate was thus easily over 25 percent for the 17 months of the war; the total casualties may have reached 35-40 percent if later injury- and disease-related deaths are added. In this respect the war was the most disastrous in American military history.

During the war political quarrels arose regarding the disposition of conquered Mexico. A strong "All-Mexico" movement urged annexation of the entire territory. Abolitionists opposed that position and fought for the exclusion of slavery from any territory absorbed by the United States. In 1847 the House of Representatives passed the Wilmot Proviso, stipulating that none of the territory acquired should be open to slavery. The Senate avoided the issue, and a late attempt to add it to the Treaty of Guadalupe Hidalgo was defeated.

The Treaty of Guadalupe Hidalgo was the unsatisfactory result of Nicholas Trist's unauthorized negotiations. It was reluctantly approved by the U.S. Senate, and eventually ratified by the Mexican Congress. Mexico was forced to agree to give up all claims for California and New Mexico, and recognize the U.S. sovereignty over all Texas north of the Rio Grande. This added 1.2 million square miles of territory to the United States. In return the United States agreed to pay $15 million and assumed the claims of its citizens against Mexico. A final territorial adjustment between Mexico and the United States was made by the Gadsden Purchase in 1853.

Questions to answer in your Study Guide: 8, 13, 14f, 14i, 14j

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