A painting of the Battle of Veracruz by Carl Nebel.
|James K. Polk
Stephen W. Kearny
John D. Sloat
|Antonio López de Santa Anna
Pedro de Ampudia
José María Flores
|78,700 soldiers||25,000–40,000 soldiers|
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The Mexican–American War was an armed conflict between the United States and Mexico from 1846 to 1848 in the wake of the 1845 U.S. annexation of Texas, which Mexico considered part of its territory in spite of the 1836 Texas Revolution.
In the U.S. the conflict is often referred to simply as the Mexican War and sometimes as the U.S.–Mexican War. In Mexico, terms for it include (primera) intervención estadounidense en México ((first) American intervention in Mexico), invasión estadounidense de México (American[a] Invasion of Mexico), and guerra del 47 (The War of '47).
Territorial expansion of the United States on the Pacific coast was foremost in the minds of President Polk and his associates in their whole conduct of the war. The major consequence of the war was the Mexican Cession of the territories of Alta California and Santa Fe de Nuevo México to the United States under the Treaty of Guadalupe Hidalgo. In addition, Mexico accepted the Rio Grande as the national boundary, and the loss of Texas.
In August 1835, President Andrew Jackson developed a "passion" to acquire all Mexican territory north of the 37th parallel north after a navy purser's favorable report on the San Francisco Bay Area, and issued instructions to pursue this, but the suggestion came to nothing.
In 1842, the American minister in Mexico Waddy Thompson, Jr. suggested Mexico might be willing to cede California to settle debts, saying "As to Texas I regard it as of very little value compared with California, the richest, the most beautiful and the healthiest country in the world... with the acquisition of Upper California we should have the same ascendency on the Pacific... France and England both have had their eyes upon it." President Tyler's administration suggested a tripartite arrangement combining a settlement of the Oregon question with cession of the port of San Francisco; Lord Aberdeen declined to participate but said Britain had no objection to U.S. territorial acquisition there.
On his part, the British minister in Mexico Richard Pakenham wrote in 1841 to Lord Palmerston urging "to establish an English population in the magnificent Territory of Upper California", saying that "no part of the World offering greater natural advantages for the establishment of an English colony ... by all means desirable ... that California, once ceasing to belong to Mexico, should not fall into the hands of any power but England ... daring and adventurous speculators in the United States have already turned their thoughts in this direction", but by the time the letter reached London, Sir Robert Peel's Tory government with a Little England policy had come to power and rejected the plan as expensive and a potential source of conflict.
In 1823, Moses Austin, a banker from Missouri, was granted a large tract of land in Texas, but died before he could bring his plan of recruiting American settlers for the land to fruition. His son, Stephen Austin, succeeded and brought over 300 families into Texas, which started the steady trend of American migration into the Texas frontier. In 1829, however, due to the large influx of American immigrants, the Americans outnumbered Mexicans in the Texas territory. The Mexican government decided to outlaw slavery and convert all immigrants to Roman Catholicism. Many, if not all, settlers refused to obey, which led to Mexico closing Texas to additional immigration from the Americas. However, citizens from the southern states continued to flow into the Texas territory.
In 1834, General Antonio Lopez de Santa Anna became the dictator of Mexico, abandoning the federal system. He decided to enforce the Mexican laws in Texas, which led to American immigrants in Texas declaring independence from Mexico in 1836, and in the following years, Texas consolidated its status as an independent republic by establishing diplomatic ties with Britain, France, and the United States. Most Texans were in favor of annexation by the United States, but U.S. President Martin Van Buren rejected it; then the pro-independence Mirabeau Lamar was president of Texas 1838-41; then the U.S. Senate rejected an annexation treaty in 1844.
Under U.S. President John Tyler, Texas was offered admission to the Union as a state via, controversially, a joint resolution of Congress rather than a treaty. The bill was signed into law on March 1, 1845. It was ratified by Texas on July 4. Texas became the 28th state on December 29, a law signed by President James K. Polk.
The Mexican government had long warned the United States that annexation would mean war. Because the Mexican congress had refused to recognize Texan independence, it saw Texas as a rebellious territory that would be retaken in the future. Britain and France, which recognized the independence of Texas, repeatedly tried to dissuade Mexico from declaring war. British efforts to mediate were fruitless, in part because additional political disputes (particularly the Oregon boundary dispute) arose between Britain and the United States. When Texas joined the U.S. as a state in 1845, the Mexican government broke diplomatic relations with the United States.
The Texan claim to the Rio Grande boundary had been omitted from the annexation resolution to help secure passage after the annexation treaty failed in the Senate, but President Polk nevertheless now claimed the Rio Grande boundary, and this provoked a dispute with Mexico. In June 1845, Polk sent General Zachary Taylor to Texas, and by October, 3,500 Americans were on the Nueces River, prepared to defend Texas from a Mexican invasion. Polk wanted to protect the border and also coveted the continent clear to the Pacific Ocean. Polk had instructed the Pacific naval squadron to seize the California ports in case Mexico declared war, while staying on good terms with the inhabitants. At the same time he wrote to Thomas Larkin, the American consul in Monterey, disclaiming American ambitions but offering to support independence from Mexico or voluntary accession to the United States, and warning that a British or French takeover would be opposed.
To end another war scare (Fifty-Four Forty or Fight) with Britain over Oregon Country, Polk retreated from the 1844 Democratic platform claiming all of it and on June 15, 1846 signed the Oregon Treaty dividing the territory, angering northern Democrats who felt he was prioritizing Southern expansion over Northern expansion.
In the winter of 1845-46, the federally commissioned explorer John C. Fremont and a group of armed men appeared in California. After telling the Mexican governor and Larkin he was merely buying supplies on the way to Oregon, he instead entered the populated part of California and visited Santa Cruz and the Salinas Valley, explaining he was looking for a seaside home for his mother. The Mexican authorities became alarmed and ordered him to leave. Fremont responded by building a fort on Gavilan Peak and raising the American flag. Larkin sent word that his actions were counterproductive. Fremont left California in March but returned to California and assisted the Bear Flag Revolt in Sonoma, where a number of American immigrants stated that they were playing “the Texas game” and declared California’s independence from Mexico.
On November 10, 1845, Polk sent John Slidell, a secret representative, to Mexico City with an offer of $25 million ($626,538,462 today) for the Rio Grande border in Texas and Mexico’s provinces of Alta California and Santa Fe de Nuevo México. U.S. expansionists wanted California to thwart British ambitions in the area and to gain a port on the Pacific Ocean. Polk authorized Slidell to forgive the $3 million ($75,184,615 today) owed to U.S. citizens for damages caused by the Mexican War of Independence and pay another $25 to $30 million ($626,538,462 to $751,846,154 today) in exchange for the two territories.
Mexico was not inclined nor in a position to negotiate. In 1846 alone, the presidency changed hands four times, the war ministry six times, and the finance ministry sixteen times. However, Mexican public opinion and all political factions agreed that selling the territories to the United States would tarnish the national honor. Mexicans who opposed open conflict with the United States, including President José Joaquín de Herrera, were viewed as traitors. Military opponents of de Herrera, supported by populist newspapers, considered Slidell's presence in Mexico City an insult. When de Herrera considered receiving Slidell in order to peacefully negotiate the problem of Texas annexation, he was accused of treason and deposed. After a more nationalistic government under General Mariano Paredes y Arrillaga came to power, it publicly reaffirmed Mexico's claim to Texas; Slidell, convinced that Mexico should be "chastised," returned to the United States.
James K. Polk ordered General Taylor and his forces south to the Rio Grande, entering the territory that Mexicans claimed as their own. Mexico claimed the Nueces River — about 150 miles (240 km) north of the Rio Grande — as its border with Texas; the United States claimed it was the Rio Grande, citing the 1836 Treaties of Velasco. Mexico, however, had never ratified these treaties, which were signed by Santa Anna while he was a prisoner in Texas. Taylor ignored Mexican demands to withdraw to the Nueces. He constructed a makeshift fort (later known as Fort Brown/Fort Texas) on the banks of the Rio Grande opposite the city of Matamoros, Tamaulipas. Mexican forces under General Mariano Arista prepared for war.
On April 25, 1846, a 2,000-strong Mexican cavalry detachment attacked a 70-man U.S. patrol that had been sent into the contested territory north of the Rio Grande and south of the Nueces River. The Mexican cavalry routed the patrol, killing 16 U.S. soldiers in what later became known as the Thornton Affair after Captain Thornton who was in command. A few survivors were returned to Fort Brown by the Mexicans, including wounded sent in an ambulance.
By then, Polk had received word of the Thornton Affair. This, added to the Mexican government's rejection of Slidell, Polk believed, constituted a casus belli (case for war). His message to Congress on May 11, 1846 stated that “Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon American soil.” Congress approved the declaration of war on May 13, with southern Democrats in strong support. Sixty-seven Whigs voted against the war on a key slavery amendment, but on the final passage only 14 Whigs voted no, including Rep. John Quincy Adams. Congress declared war on Mexico on May 13, 1846 after only having a few hours to debate. Although President Paredes's issuance of a manifesto on May 23 is sometimes considered the declaration of war, Mexico officially declared war by Congress on July 7.
Once the United States declared war on Mexico, Antonio Lopez de Santa Anna wrote to Mexico City saying he no longer had aspirations to the presidency, but would eagerly use his military experience to fight off the foreign invasion of Mexico as he had in the past. President Valentín Gómez Farías was desperate enough to accept the offer and allowed Santa Anna to return. Meanwhile, Santa Anna had secretly been dealing with representatives of the United States, pledging that if he were allowed back in Mexico through the U.S. naval blockades, he would work to sell all contested territory to the United States at a reasonable price. Once back in Mexico at the head of an army, Santa Anna reneged on both of these agreements. Santa Anna declared himself president again and unsuccessfully tried to fight off the United States invasion.
In the United States, increasingly divided by sectional rivalry, the war was a partisan issue and a key part of the origins of the American Civil War. Most Whigs in the North and South opposed it; most Democrats supported it. Southern Democrats, animated by a popular belief in Manifest Destiny, supported it in hopes of adding territory to the South and avoiding being outnumbered by the faster-growing North. It was John O'Sullivan, the editor of the "Democratic Review", who coined this phrase in its context, stating that it must be "Our manifest destiny to overspread the continent alloted by Providence for the free development of our yearly multiplying millions." Northern anti-slavery elements feared the growth of a Slave Power; Whigs generally wanted to deepen the economy with industrialization, not expand it with more land. Democrats wanted more land, and northern Democrats were especially attracted by the possibilities in the far northwest. Joshua Giddings led a group of dissenters in Washington D.C. He called the war with Mexico "an aggressive, unholy, and unjust war," and voted against supplying soldiers and weapons. He said:
|“||In the murder of Mexicans upon their own soil, or in robbing them of their country, I can take no part either now or here-after. The guilt of these crimes must rest on others. I will not participate in them.||”|
Fellow Whig Abraham Lincoln, who had been elected to Congress several months after the declaration of war, contested the causes for the war and demanded to know exactly where Thornton had been attacked and American blood shed. "Show me the spot," he demanded. Whig leader Robert Toombs of Georgia declared:
|“||This war is nondescript.... We charge the President with usurping the war-making power... with seizing a country... which had been for centuries, and was then in the possession of the Mexicans.... Let us put a check upon this lust of dominion. We had territory enough, Heaven knew."||”|
Northern abolitionists attacked the war as an attempt by slave-owners—frequently referred to as "the Slave Power" — to expand the grip of slavery and thus assure their continued influence in the federal government. Acting on his convictions, Henry David Thoreau was jailed for his refusal to pay taxes to support the war, and penned his famous essay, Civil Disobedience.
Former President John Quincy Adams also expressed his belief that the war was fundamentally an effort to expand slavery in a speech he gave before the House on May 25, 1846. In response to such concerns, Democratic Congressman David Wilmot introduced the Wilmot Proviso, which aimed to prohibit slavery in any new territory acquired from Mexico. Wilmot's proposal did not pass Congress, but it spurred further hostility between the factions.
President Polk strongly countered these arguments in his Third Annual Message on December 7, 1847  going into length to explain the origins of the conflict, the measures the United States had gone to in avoiding the war, and the justification for declaring the war. This helped to rally Congressional Democrats to his side ensuring passage of his war measures and bolstering support for the war in the United States.
The Siege of Fort Texas began on May 3. Mexican artillery at Matamoros opened fire on Fort Texas, which replied with its own guns. The bombardment continued for 160 hours and expanded as Mexican forces gradually surrounded the fort. Thirteen U.S. soldiers were injured and two killed during the bombardment. Among the dead was Jacob Brown, after whom the fort was later named.
On May 8, Zachary Taylor arrived with 2,400 troops to relieve the fort. However, Arista rushed north and intercepted him with a force of 3,400 at Palo Alto. The Americans employed "flying artillery," the American term for horse artillery, a type of mobile light artillery that was mounted on horse carriages with the entire crew riding horses into battle. It had a devastating effect on the Mexican army. The Mexicans replied with cavalry skirmishes and their own artillery. The U.S. flying artillery somewhat demoralized the Mexican side, and seeking terrain more to their advantage, the Mexicans retreated to the far side of a dry riverbed (resaca) during the night. It provided a natural fortification, but during the retreat, Mexican troops were scattered, making communication difficult. During the Battle of Resaca de la Palma the next day, the two sides engaged in vicious hand to hand combat. The U.S. cavalry managed to capture the Mexican artillery, causing the Mexican side to retreat — a retreat that turned into a rout. Fighting on unknown terrain, his troops fleeing in retreat, Arista found it impossible to rally his forces. Mexican casualties were heavy, and the Mexicans were forced to abandon their artillery and baggage. Fort Brown inflicted further casualties as the withdrawing troops passed by the fort. Many Mexican soldiers drowned trying to swim across the Rio Grande.
After the declaration of war, U.S. forces invaded Mexican territory on two main fronts. The U.S. war department sent a cavalry force under Stephen W. Kearny to invade western Mexico from Fort Leavenworth, reinforced by a Pacific fleet under John D. Sloat. This was done primarily because of concerns that Britain might also attempt to occupy the area. Two more forces, one under John E. Wool and the other under Taylor, were ordered to occupy Mexico as far south as the city of Monterrey.
Although the United States declared war against Mexico on May 13, 1846, it took almost two months (until the middle of June, 1846) for definite word of war to get to California. American consul Thomas O. Larkin, stationed in Monterey, on hearing rumors of war tried to keep peace between the States and the small Mexican military garrison commanded by José Castro. U.S. Army captain John C. Frémont, with about sixty well-armed men, had entered California in December, 1845, and was marching slowly to Oregon when he received word that war between Mexico and the U.S. was imminent and so began his chapter of the war, the "Bear Flag Revolt".
On June 15, 1846, some thirty settlers, mostly American citizens, staged a revolt and seized the small Mexican garrison in Sonoma. They raised the "Bear Flag" of the California Republic over Sonoma. The republic was in existence scarcely more than a week before the U.S. Army, led by Frémont, took over on June 23. The California state flag today is based on this original Bear Flag and still contains the words, "California Republic."
Commodore John Drake Sloat, upon hearing of imminent war and the revolt in Sonoma, ordered his naval and marine forces to occupy Yerba Buena (present-day San Francisco) on July 7 and raise the flag of the United States; this was accomplished on July 9. On July 15, Sloat transferred his command to Commodore Robert F. Stockton, a much more aggressive leader, who put Frémont's forces under his orders. On July 19, Frémont's "California Battalion" swelled to about 160 additional men from newly-arrived settlers near Sacramento, and he entered Monterey in a joint operation with some of Stockton's sailors and marines. The word had been received: war was official. The U.S. forces easily took over the north of California; within days they controlled San Francisco, Sonoma, and the privately owned Sutter's Fort in Sacramento.
From Alta California (the present-day American state of California), Mexican General José Castro and Governor Pío Pico fled southward into still-loyal Mexico. When Stockton's forces, sailing southward to San Diego, stopped in San Pedro, he sent fifty U.S. Marines ashore; this force entered Los Angeles unresisted on August 13, 1846. With the success of this so-called "Siege of Los Angeles", the nearly bloodless conquest of California seemed complete.
Stockton, however, left too small a force in Los Angeles, and the Californios, acting on their own and without help from Mexico, led by José María Flores, forced the American garrison to retreat, late in September. The rancho vaqueros who had banded together to defend their land fought as Californio lancers; they were a force the Americans had not anticipated. More than three hundred American reinforcements, sent by Stockton and led by Captain William Mervine, U.S.N., were repulsed in the Battle of Dominguez Rancho, fought from October 7 through October 9, 1846, near San Pedro. Fourteen American Marines were killed.
Meanwhile, General Stephen W. Kearny, with a squadron of 139 dragoons that he had led on a grueling march across New Mexico, Arizona, and the Sonoran desert, finally reached California on December 6, 1846, and fought in a small battle with Californio lancers at the Battle of San Pasqual near San Diego, California, where 22 of Kearny's troops were killed.
Kearny's command was bloodied and in poor condition but pushed on until they had to establish a defensive position on "Mule" Hill near present-day Escondido. The Californios besieged the dragoons for four days until Commodore Stockton's relief force arrived. The resupplied, combined American force marched north from San Diego on December 29 and entered the Los Angeles area on January 8, 1847, linking up with Frémont's men there. American forces now totalling 607 soldiers and marines fought and defeated a Californio force of about 300 men under the command of Captain-general Flores in the decisive Battle of Rio San Gabriel. The next day, January 9, 1847, the Americans fought and won the Battle of La Mesa. On January 12, the last significant body of Californios surrendered to U.S. forces. That marked the end of armed resistance in California, and the Treaty of Cahuenga was signed the next day, on January 13, 1847.
The defeats at Palo Alto and Resaca de la Palma caused political turmoil in Mexico, turmoil which Antonio López de Santa Anna used to revive his political career and return from self-imposed exile in Cuba in mid-August 1846. He promised the U.S. that if allowed to pass through the blockade, he would negotiate a peaceful conclusion to the war and sell the New Mexico and Alta California territories to the United States. Once Santa Anna arrived in Mexico City, however, he reneged and offered his services to the Mexican government. Then, after being appointed commanding general, he reneged again and seized the presidency.
Led by Taylor, 2,300 U.S. troops crossed the Rio Grande (Rio Bravo) after some initial difficulties in obtaining river transport. His soldiers occupied the city of Matamoros, then Camargo (where the soldiery suffered the first of many problems with disease) and then proceeded south and besieged the city of Monterrey. The hard-fought Battle of Monterrey resulted in serious losses on both sides. The American light artillery was ineffective against the stone fortifications of the city. The Mexican forces were under General Pedro de Ampudia. A U.S. infantry division and the Texas Rangers captured four hills to the west of the town and with them heavy cannon. That lent the U.S. soldiers the strength to storm the city from the west and east. Once in the city, U.S. soldiers fought house to house: each was cleared by throwing lighted shells, which worked like grenades.
Eventually, these actions drove and trapped Ampudia's men into the city's central plaza, where howitzer shelling forced Ampudia to negotiate. Taylor agreed to allow the Mexican Army to evacuate and to an eight-week armistice in return for the surrender of the city. Under pressure from Washington, Taylor broke the armistice and occupied the city of Saltillo, southwest of Monterrey. Santa Anna blamed the loss of Monterrey and Saltillo on Ampudia and demoted him to command a small artillery battalion. On February 22, 1847, Santa Anna personally marched north to fight Taylor with 20,000 men. Taylor, with 4,600 men, had entrenched at a mountain pass called Buena Vista. Santa Anna suffered desertions on the way north and arrived with 15,000 men in a tired state. He demanded and was refused surrender of the U.S. army; he attacked the next morning. Santa Anna flanked the U.S. positions by sending his cavalry and some of his infantry up the steep terrain that made up one side of the pass, while a division of infantry attacked frontally along the road leading to Buena Vista. Furious fighting ensued, during which the U.S troops were nearly routed, but managed to cling to their entrenched position. The Mexicans had inflicted considerable losses but Santa Anna had gotten word of upheaval in Mexico City,so he withdrew that night, leaving Taylor in control of part of Northern Mexico. Polk distrusted Taylor, whom he felt had shown incompetence in the Battle of Monterrey by agreeing to the armistice, and may have considered him a political rival for the White House. Taylor later used the Battle of Buena Vista as the centerpiece of his successful 1848 presidential campaign.
On March 1, 1847, Alexander William Doniphan occupied Chihuahua City. He found the inhabitants much less willing to accept the American conquest than the New Mexicans. The British consul John Potts did not want to let Doniphan search Governor Trias's mansion and unsuccessfully asserted it was under British protection. American merchants in Chihuahua wanted the American force to stay in order to protect their business. Gilpin advocated a march on Mexico City and convinced a majority of officers, but Doniphan subverted this plan, then in late April Taylor ordered the First Missouri Mounted Volunteers to leave Chihuahua and join him at Saltillo. The American merchants either followed or returned to Santa Fe. Along the way the townspeople of Parras enlisted Doniphan's aid against an Indian raiding party that had taken children, horses, mules, and money.
During the war inventions such as the telegraph created new communication ways that updated people with the latest news from the reporters, who were usually on the scene. With more than a decade’s experience reporting urban crime, the “penny press”, was able to realize the voracious need of the public to get the astounding war news. This was the very first time in the American history where the accounts by journalists, instead of the opinions of politicians, caused great influence in shaping people’s minds and attitudes toward a war. At all times, news about the war caused extraordinary popular excitement.
By getting constant reports from the battlefield, Americans became emotionally united as a community. In the spring of 1846 news about Zachary Taylor's victory at Palo Alto brought up a large crowd that met in a cotton textile town of Lowell, Massachusetts. At Veracruz and Buena Vista, New York celebrated their twin victories in May 1847. Among fireworks and illuminations, they had a “grand procession” of about 400,000 people. Generals Taylor and Scott became heroes for their people and later became presidential candidates.
The desertion rate was a major problem for the Mexican army, depleting forces on the eve of battle. Most of the soldiers were peasants who had a loyalty to their village and family, but not to the generals who conscripted them. Often hungry and ill, never well paid, under equipped and only partially trained, the soldiers were held in contempt by their officers and had little reason to fight the Americans. Looking for their opportunity, many slipped away from camp to find their way back to their home village.
The desertion rate in the U.S. army was 8.3 percent (9,200 out of 111,000), compared to 12.7 percent during the War of 1812 and usual peacetime rates of about 14.8 percent per year. Many men deserted in order to join another U.S. unit and get a second enlistment bonus. Others deserted because of the miserable conditions in camp, or used the army to get free transportation to California, where they deserted to join the gold rush.
Several hundred deserters went over to the Mexican side; nearly all were recent immigrants from Europe with weak ties to the U.S., the most famous group being Saint Patrick's Battalion, about half of whom were Catholics from Ireland. The Mexicans issued broadsides and leaflets enticing U.S. soldiers with promises of money, land bounties, and officers' commissions. Mexican guerrillas shadowed the U.S. Army and captured men who took unauthorized leave or fell out of the ranks. The guerrillas coerced these men to join the Mexican ranks The generous promises proved illusory for most deserters, who risked getting shot if captured by U.S. forces. Indeed, about fifty of the San Patricios were tried and hanged following their capture at Churubusco in August 1847.
Rather than reinforce Taylor's army for a continued advance, President Polk sent a second army under General Winfield Scott, which was transported to the port of Veracruz by sea, to begin an invasion of the Mexican heartland. Scott performed the first major amphibious landing in the history of the United States in preparation for the Siege of Veracruz. A group of 12,000 volunteer and regular soldiers successfully offloaded supplies, weapons and horses near the walled city. Included in the invading force were Robert E. Lee, George Meade, Ulysses S. Grant, and Thomas "Stonewall" Jackson. The city was defended by Mexican General Juan Morales with 3,400 men. Mortars and naval guns under Commodore Matthew C. Perry were used to reduce the city walls and harass defenders. The city replied as best as it could with its own artillery. The effect of the extended barrage destroyed the will of the Mexican side to fight against a numerically superior force, and they surrendered the city after 12 days under siege. U.S. troops suffered 80 casualties, while the Mexican side had around 180 killed and wounded, about half of whom were civilian. During the siege, the U.S. side began to fall victim to yellow fever.
Scott then marched westward toward Mexico City with 8,500 healthy troops, while Santa Anna set up a defensive position in a canyon around the main road at the halfway mark to Mexico City, near the hamlet of Cerro Gordo. Santa Anna had entrenched with 12,000 troops and artillery that were trained on the road, along which he expected Scott to appear. However, Scott had sent 2,600 mounted dragoons ahead, and the Mexican artillery prematurely fired on them and revealed their positions. Instead of taking the main road, Scott's troops trekked through the rough terrain to the north, setting up his artillery on the high ground and quietly flanking the Mexicans. Although by then aware of the positions of U.S. troops, Santa Anna and his troops were unprepared for the onslaught that followed. The Mexican army was routed. The U.S. army suffered 400 casualties, while the Mexicans suffered over 1,000 casualties and 3,000 were taken prisoner. In August 1847, Captain Kirby Smith, of Scott's 3rd Infantry, reflected on the resistance of the Mexican army:
|“||What stupid people they are! They can do nothing and their continued defeats should convince them of it. They have lost six great battles; we have captured six hundred and eight cannon, nearly one hundred thousand stands of arms, made twenty thousand prisoners, have the greatest portion of their country and are fast advancing on their Capital which must be ours,—yet they refuse to treat [ie negotiate terms]!||”|
In May, Scott pushed on to Puebla, the second largest city in Mexico. Because of the citizens' hostility to Santa Anna, the city capitulated without resistance on May 1. Mexico City was laid open in the Battle of Chapultepec and subsequently occupied. Winfield Scott became an American national hero after his victories in the Mexican–American War, and later became military governor of occupied Mexico City.
Outnumbered militarily and with many of its large cities occupied, Mexico could not defend itself and was also faced with internal divisions. It had little choice but to make peace on any terms. The Treaty of Guadalupe Hidalgo, signed on February 2, 1848, by American diplomat Nicholas Trist and Mexican plenipotentiary representatives Luis G. Cuevas, Bernardo Couto, and Miguel Atristain, ended the war and gave the U.S. undisputed control of Texas, established the U.S.-Mexican border of the Rio Grande River, and ceded to the United States the present-day states of California, Nevada, Utah, and parts of Colorado, Arizona, New Mexico, and Wyoming. In return Mexico received US $18,250,000 ($457,373,077 today)—less than half the amount the U.S. had attempted to offer Mexico for the land before the opening of hostilities—and the U.S. agreed to assume $3.25-million ($81,450,000 today) in debts that the Mexican government owed to U.S. citizens.
The acquisition was a source of controversy at the time, especially among U.S. politicians who had opposed the war from the start. A leading antiwar U.S. newspaper, the Whig Intelligencer sardonically concluded that:
|“||We take nothing by conquest.... Thank God.||”|
Jefferson Davis introduced an amendment giving the U.S. most of northeastern Mexico, which failed 44-11. It was supported by both senators from Texas (Sam Houston and Thomas Jefferson Rusk), Daniel S. Dickinson of New York, Stephen A. Douglas of Illinois, Edward A. Hannegan of Indiana, and one each from Alabama, Florida, Mississippi, Ohio, Missouri, and Tennessee. Most of the leaders of the Democratic party, Thomas Hart Benton, John C. Calhoun, Herschel V. Johnson, Lewis Cass, James Murray Mason of Virginia, and Ambrose Hundley Sevier, were opposed. An amendment by Whig Senator George Edmund Badger of North Carolina to exclude New Mexico and California lost 35-15, with three Southern Whigs voting with the Democrats. Daniel Webster was bitter that four New England senators made deciding votes for acquiring the new territories.
The acquired lands west of the Rio Grande are traditionally called the Mexican Cession in the United States, as opposed to the Texas Annexation two years earlier, though division of New Mexico down the middle at the Rio Grande never had any basis either in actual control or in Mexican boundaries. Mexico never recognized the independence of Texas  prior to the war, and did not cede its claim to territory north of the Rio Grande or Gila River until this treaty.
Prior to ratifying the treaty, the U.S. Senate made two modifications, changing the wording of Article IX (which guaranteed Mexicans living in the purchased territories the right to become U.S. citizens), and striking out Article X (which conceded the legitimacy of land grants made by the Mexican government). On May 26, 1848, when the two countries exchanged ratifications of the treaty of Guadalupe Hidalgo, they further agreed to a three-article protocol (known as the Protocol of Querétaro) to explain the amendments. The first article claimed that the original Article IX of the treaty, although replaced by Article III of the Treaty of Louisiana, would still confer the rights delineated in Article IX. The second article confirmed the legitimacy of land grants pursuant to Mexican law. The protocol was signed in the city of Querétaro by A. H. Sevier, Nathan Clifford, and Luis de la Rosa.
Mexico lost more than 500,000 square miles (about 1,300,000 km²) of land, 55% of its national territory. This figure rises to over two thirds of its territory if Texas is included. The annexed territories contained about 1,000 Mexican families in Alta California and 7,000 in Nuevo México. A few relocated further south in Mexico; the great majority remained in the United States. Descendants of these Mexican families have risen to prominence in American life, such as United States Secretary of the Interior Ken Salazar, and his brother, U.S. Rep. John Salazar, both from Colorado.
A month before the end of the war, Polk was criticized in a United States House of Representatives amendment to a bill praising Major General Zachary Taylor for "a war unnecessarily and unconstitutionally begun by the President of the United States". This criticism, in which Congressman Abraham Lincoln played an important role with his Spot Resolutions, followed congressional scrutiny of the war's beginnings, including factual challenges to claims made by President Polk. The vote followed party lines, with all Whigs supporting the amendment. Lincoln's attack won luke-warm support from fellow Whigs in Illinois but was harshly counter-attacked by Democrats, who rallied pro-war sentiments in Illinois; Lincoln's Spot resolutions haunted his future campaigns in the heavily Democratic state of Illinois, and was cited by enemies well into his presidency.
In much of the U.S., victory and the acquisition of new land brought a surge of patriotism (the country had also acquired the southern half of the Oregon Country in 1846 through a treaty with Great Britain). Victory seemed to fulfill citizens' belief in their country's Manifest Destiny. While Whig Ralph Waldo Emerson rejected war "as a means of achieving America's destiny," he accepted that "most of the great results of history are brought about by discreditable means". Although the Whigs had opposed the war, they made Zachary Taylor their presidential candidate in the election of 1848, praising his military performance while muting their criticism of the war itself.
Many of the military leaders on both sides of the American Civil War had fought as junior officers in Mexico, including Ulysses S. Grant, George B. McClellan, Ambrose Burnside, Stonewall Jackson, James Longstreet, George Meade, and Robert E. Lee, as well as the future Confederate President Jefferson Davis.
In Mexico City's Chapultepec Park, the Monument to the Heroic Cadets commemorates the heroic sacrifice of six teenaged military cadets who fought to their deaths rather than surrender to American troops during the Battle of Chapultepec Castle on September 18, 1847. The monument is an important patriotic site in Mexico. On March 5, 1947, nearly one hundred years after the battle, U.S. President Harry S. Truman placed a wreath at the monument and stood for a moment of silence.
|“||Generally, the officers of the army were indifferent whether the annexation was consummated or not; but not so all of them. For myself, I was bitterly opposed to the measure, and to this day regard the war, which resulted, as 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.||”|
Grant also expressed the view that the war against Mexico had brought punishment on the United States in the form of the American Civil War:
|“||The Southern rebellion was largely the outgrowth of the Mexican war. Nations, like individuals, are punished for their transgressions. We got our punishment in the most sanguinary and expensive war of modern times.||”|
On the American side, the war was fought by regiments of regulars and various regiments, battalions, and companies of volunteers from the different states of the union and the Americans and some of the Mexicans in the territory of California and New Mexico. On the West Coast the U. S. Navy fielded a battalion in an attempt to recapture Los Angeles.
At the beginning of the war the United States Army had eight regiments of infantry (three battalions), four artillery regiments and three mounted regiments (two dragoons, one of mounted rifles). These regiments were supplemented by 10 new regiments (nine of infantry and one of cavalry) raised for one year's service (new regiments raised for one year according to act of Congress Feb. 11, 1847). 
State Volunteers were raised in various sized units and for various periods of time, mostly for one year. Later some were raised for the duration of the war as it became clear it was going to last longer than a year. 
U.S. soldiers' memoirs describe cases of scalping innocent civilians, the rape and murder of women, the murder of children, the burning of homes, and the desecrating of Catholic religious objects and buildings. One officer's diary records:
|“||We reached Burrita about 5 pm, many of the Louisiana volunteers were there, a lawless drunken rabble. They had driven away the inhabitants, taken possession of their houses, and were emulating each other in making beasts of themselves.||”|
John L. O'Sullivan, a vocal proponent of Manifest Destiny, later recollected:
|“||The regulars regarded the volunteers with importance and contempt ... [The volunteers] robbed Mexicans of their cattle and corn, stole their fences for firewood, got drunk, and killed several inoffensive inhabitants of the town in the streets.||”|
Many of the volunteers were unwanted and considered poor soldiers. The expression "Just like Gaines's army" came to refer to something useless, the phrase having originated when a group of untrained and unwilling Louisiana troops were rejected and sent back by Gen. Taylor at the beginning of the war.
At the beginning of the war, Mexican forces were divided between the permanent forces (permanentes) and the active militiamen (activos). The permanent forces consisted of 12 regiments of infantry (of two battalions each), three brigades of artillery, eight regiments of cavalry, one separate squadron and a brigade of dragoons. The militia amounted to nine infantry and six cavalry regiments. In the northern territories of Mexico presidial companies (presidiales) protected the scattered settlements there.
One of the contributing factors to loss of the war by Mexico was the inferiority of their weapons. The Mexican army was using British muskets (e.g. Brown Bess) from the Napoleonic Wars; furthermore, Mexican troops were trained to fire with their muskets held loosely at hip-level, while U.S. soldiers used the more accurate method of butting the rifle up to the shoulder and taking aim along the barrel. In contrast to the aging Mexican standard-issue infantry weapon, some U.S. troops had the latest U.S.-manufactured breech-loading Hall rifles and Model 1841 percussion rifles. In the later stages of the war, U.S. cavalry and officers were issued Colt Walker revolvers, of which the U.S. Army had ordered 1,000 in 1846. Throughout the war, the superiority of the U.S. artillery often carried the day.
Political divisions inside Mexico were another factor in the U.S. victory. Inside Mexico, the centralistas and republicans vied for power, and at times these two factions inside Mexico's military fought each other rather than the invading American army. Another faction called the monarchists, whose members wanted to install a monarch (some even advocated rejoining Spain), further complicated matters. This third faction would rise to predominance in the period of the French intervention in Mexico.
Saint Patrick's Battalion (San Patricios) was a group of several hundred immigrant soldiers, the majority Irish, who deserted the U.S. Army because of ill-treatment or sympathetic leanings to fellow Mexican Catholics. They joined the Mexican army. Most were killed in the Battle of Churubusco; about 100 were captured by the U.S. and roughly half were hanged as deserters.
Despite initial objections from the Whigs and abolitionists, the war would nevertheless unite the United States in a common cause and was fought almost entirely by volunteers. The army swelled from just over 6,000 to more than 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–American 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 proportionately the most deadly in American military history.
During the war, political quarrels in the U.S. arose regarding the disposition of conquered Mexico. A brief "All-Mexico" movement urged annexation of the entire territory. Veterans of the war who had seen Mexico first hand were unenthusiastic. Anti-slavery elements 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 result of Nicholas Trist's unauthorized negotiations. It was approved by the U.S. Senate on March 10, 1848, and ratified by the Mexican Congress on May 25. Mexico's cession of Alta California and Nuevo México and its recognition of U.S. sovereignty over all of Texas north of the Rio Grande formalized the addition of 1.2-million square miles (3.1-million km²) 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.
As late as 1880 the "Republican Campaign Textbook" by the Republican Congressional Committee described the war as "Feculent, reeking Corruption" and "one of the darkest scenes in our history - a war forced upon our and the Mexican people by the high-handed usurpations of Pres't Polk in pursuit of territorial aggrandizement of the slave oligarchy".