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. Mexico claimed ownership of Texas as a breakaway province and refused to recognize the secession and subsequent military victory by Texas in 1836.
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 intervención norteamericana en México (North American intervention in Mexico), invasión estadounidense de México (U.S. invasion of Mexico), and guerra del 47 (war of '47).
The most important consequence of the war for the United States was the Mexican Cession, in which the Mexican territories of Alta California and Santa Fé de Nuevo México were ceded to the United States under the terms of the Treaty of Guadalupe Hidalgo. In Mexico, the enormous loss of territory following the war encouraged its government to enact policies to colonize its remaining northern territories as a hedge against further losses. In addition the Rio Grande/Rio Bravo river became the boundary between Texas and Mexico, and Mexico never again claimed ownership of Texas.
Seeking to better control the border region of Texas, which had few settlers, the Mexican government permitted a few hundred U.S. families to settle in the area. This, however, led to settlement of Texas on a scale unanticipated by the Mexican government, as its inability to control the border allowed thousands more Americans to settle than had been agreed upon. English speakers quickly formed a majority in Texas.
Although the United States made overtures to the Mexican government to buy Texas, the short-lived régime of Emperor Agustín Iturbide and then his successor, Antonio López de Santa Anna, staunchly opposed selling any territory. Mexico instead intended to colonize its northern provinces with Spanish-speaking settlers.
Texians had become increasingly disillusioned with the Mexican government. Many Mexican soldiers garrisoned in Texas were convicted criminals who had been given the choice of prison or serving in the army in Texas. Many Texians were also unhappy with the location of their state capital, which moved periodically between Saltillo and Monclova, both of which were in southern Coahuila, some 500 miles (800 km) away; they wanted Texas to be a separate state from Coahuila (but not independent from Mexico) and to have its own capital. They believed a closer capital would help to stem corruption and facilitate other matters of government. Some citizens were accustomed to the rights they had in the U.S. that they did not have in Mexico.
Mexico did not protect freedom of religion, instead requiring colonists to pledge their acceptance of Roman Catholicism. Also, there was discontent with the deal Stephen Austin made with the Mexican government whereby farmers and ranchers had to offer their products first to Mexico before other markets. Cotton was in great demand throughout Europe and most settlers wanted to raise cotton for big profits. However, Mexico demanded that the settlers produce corn, grain, and beef and dictated which crops each settler would plant. Unlike the states of the southern United States, where slavery was legal, Mexico had abolished slavery in 1829. The Mexican government made little effort to enforce abolition; however, periodic threats from the Mexican federal government and the state government of Coahuila y Tejas incensed slave-holding Texians.
In 1836, Texas had an estimated population of 38,470, including 5000 slaves. Texas and other states in Mexico were further incensed in 1836 when General Santa Anna abolished the 1824 constitution, replacing it with one that further centralized power in Mexico City. The new centralist constitution enshrined the Siete Leyes (Seven Laws), which included secular reforms but granted additional powers to the president, such as the power to close Congress and suppress the judiciary. Several states rebelled against the new central government under Santa Anna, including Texas (then a department of the state of Coahuila y Tejas), San Luis Potosí, Querétaro, Durango, Guanajuato, Michoacán, Jalisco and Zacatecas. Coahuila, Nuevo León, and Tamaulipas withdrew from Mexico and established the Republic of the Rio Grande in 1840, which was defeated by Santa Anna that same year. The Republic of Yucatán also withdrew from Mexico later, and declared its neutrality in the Mexican-American War.
The Texas Revolution erupted in 1836, after Texas declared its independence from Mexico. Mexico responded by invading Texas. General Santa Anna won major victories in the battles of the Alamo and Goliad.
On April 21, 1836, the Texans decisively defeated Santa Anna's forces in the Battle of San Jacinto. Santa Anna himself was taken captive by the Texas militia and released after signing the Treaties of Velasco, in which he promised to recognize the sovereignty of the Republic of Texas and the Rio Grande as the boundary between Texas and Mexico. The Mexican government, however, refused to acknowledge these concessions, arguing that Santa Anna was not a representative of Mexico, that he had no authority to negotiate on behalf of Mexico, and that he signed away Texas under duress. The Mexican government never ratified the treaties.
In the years after 1836, Texas consolidated its position 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 Andrew Jackson rejected it.
Under U.S. President John Tyler, Texas was offered admission to the Union as a slave state. 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 United States supported Texas when it claimed all land north of the Rio Grande, and this provoked a dispute with Mexico. In June 1845, James K. Polk sent General Zachary Taylor to Texas, and by October, 3500 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 furtively instructed the Pacific naval squadron to seize the California ports in case Mexico declared war. At the same time he wrote to Thomas Larkin, the American consul in Monterey, that a peaceful takeover of California would be welcomed.
In the winter of 1845-46, the federally commissioned explorer John C. Fremont and a group of armed men appeared in California. The Mexican authorities became alarmed and ordered him to leave. Fremont returned to California and assisted the Bear Flag Revolt in Sonoma, where a number of American settlers 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 $30 million or even more for the Rio Grande border in Texas and Mexico’s provinces of Alta California and Santa Fé 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 $4,265,464 owed to U.S. citizens for damages caused by the Mexican War of Independence and pay another $25 to $30 million in exchange for the two territories. However, 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.
Polk ordered General Taylor and his forces south to the Rio Grande, invading 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. Taylor ignored Mexican demands to withdraw to the Nueces. He constructed a makeshift fort (later known as Fort Brown) 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 63-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 11 U.S. soldiers in what later became known as the Thornton Affair after the slain U.S. officer who was in command. A few survivors were returned to Fort Brown by the Mexicans, including wounded sent in an ambulance.
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. 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 a 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.
To end mounting tensions with Britain over Oregon Country, Polk negotiated a treaty with Britain that gave the U.S. control of the southern half of Oregon.
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 difficult terrain, his troops scattered, 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.
When the U.S. declared war against Mexico, on May 13, 1846, it took almost two months (mid-July 1846) for definite word of war to get to California. U.S. consul Thomas O. Larkin, stationed in Monterey, on hearing rumors of war tried to keep peace between the U.S. and the small Mexican military garrison commanded by José Castro. U.S. Army captain John C. Frémont with about 60 well-armed men had entered California in December 1845 and was making a slow march to Oregon when they received word that war between Mexico and the U.S. was imminent and so began their chapter of the war, the "Bear Flag Revolt".
On June 15, 1846, some 30 settlers, mostly U.S. citizens, staged a revolt and seized the small Mexican garrison in Sonoma. They raised the "Bear Flag" of the California Republic over Sonoma. It lasted one week until 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, on 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 U.S. flag, which 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 — the war was official. The U.S. forces easily took over the north of California; within days they controlled San Francisco, Sonoma, and Sutter's Fort in Sacramento.
In Northern California, Mexican General José Castro and Governor Pío Pico fled further south into loyalist Mexico. When Stockton's forces, sailing south to San Diego, stopped in San Pedro, he dispatched 50 U.S. Marines. The force entered Los Angeles unresisted on August 13, 1846. It is known as the 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 Mariá Flores, forced the American garrison to retreat in late September. More than 300 reinforcements sent by Stockton, led by U.S. Navy Captain William Mervine, were repulsed in the Battle of Dominguez Rancho, October 7 through October 9, 1846, near San Pedro. 14 U.S. Marines were killed. The rancho vaqueros, banded together to defend their land, fought as Californio Lancers. They were a force the Americans had not prepared for. Meanwhile, General Stephen W. Kearny, with a squadron of 139 dragoons, finally reached California after a grueling march across New Mexico, Arizona and the Sonora desert, 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. Later, their re-supplied, combined force, marched north from San Diego on December 29, entering the Los Angeles area on January 8, 1847, linking up with Frémont's men. With U.S. forces totaling 607 soldiers and marines, they 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, and the next day, January 9, 1847, they fought the Battle of La Mesa. On January 12, 1847, the last significant body of Californios surrendered to U.S. forces. That marked the end of the war in California. On January 13, 1847, the Treaty of Cahuenga was signed.
On January 28, 1847, U.S. Army Lieutenant William Tecumseh Sherman and some army units arrived in Monterey, California. The next day, the famous Mormon Battalion commanded by fellow dragoon, Lt. Col. Philip St. George Cooke arrived at San Diego after making a remarkable march from Council Bluffs, Iowa. Other U.S. forces continued to arrive in California. On March 15, 1847, Col. Jonathan D. Stevenson’s Seventh Regiment of New York Volunteers of about 900 men started arriving in California. All of these men were in place when word went out that gold was discovered in California, January 1848.
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 some U.S. troops were routed, but were saved by artillery fire against a Mexican advance at close range by Captain Braxton Bragg, and a charge by the mounted Mississippi Riflemen under Jefferson Davis. Having suffered discouraging losses and word of upheaval in Mexico City, Santa Anna withdrew that night, leaving Taylor in control 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.
The desertion rate in the U.S. army was 8.3% (9,200 out of 111,000), compared to 12.7% during the War of 1812 and usual peacetime rates of about 14.8% 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 were using the army to get free transportation to California, where they deserted to join the gold rush.
Several hundred deserters went over the to Mexican side; nearly all were recent immigrants from Europe with weak ties to the U.S. The most famous group was the 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—threatening to kill them if they failed to comply. 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.
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 a 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.
The Treaty of Guadalupe Hidalgo, signed on February 2, 1848 by American diplomat Nicholas Trist, 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 $15,000,000—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 in debts that the Mexican government owed to U.S. citizens. The acquisition was a source of controversy at time, especially among U.S. politicians who had opposed the war from the start. A leading U.S. newspaper, the Whig Intelligencer sardonically concluded that:
We take nothing by conquest.... Thank God.|The sale of land is known in the United States as the Mexican Cession.
Prior to ratifying the treaty, the U.S. Senate made two modifications, changing the language 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 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 U.S. Senator 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 haunted his future campaigns in the heavily Democratic state of Illinois, and was cited by enemies well into his presidency. The stand did not cost Lincoln his Congressional seat in Illinois' Seventh Congressional District; the district was the only place in Illinois where a Whig could win high office, and party leaders agreed to one-term limits for Whig representatives there. Lincoln was succeeded by a Democrat, but the Seventh Congressional District voted for Zachary Taylor, a Whig, that fall.
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."
"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."
In 1879, while in China during his post presidential world tour, Grant told John Russell Young: "I had very strong opinions on the subject. I do not think there was ever a more wicked war than that waged by the United States on Mexico. I had a horror of the Mexican War, and I have always believed that it was on our part most unjust. The wickedness was not in the way our soldiers conducted it, but in the conduct of our government in declaring war. We had no claim on Mexico. Texas had no claim beyond the Nueces River, and yet we pushed on to the Rio Grande and crossed it. I am always ashamed of my country when I think of that invasion" .
On the American side, the war was fought by regulars and the volunteers, with the majority of atrocities being committed by volunteers. 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 General Taylor at the beginning of the war.
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 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 flintlock "Hall's" rifles and Percussion cap Model 1841 rifles. In the later stages of the war, U.S. cavalry and officers were issued Colt revolvers, of which the U.S. army had ordered 1000 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 the two factions inside Mexico's military fought each other rather than these invading American army. Another faction called the monarchists, whose members wanted to install a king (some even advocated rejoining Spain) further complicated matters. This third faction would rise to predominance in the period of the French intervention in Mexico.
The 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.
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% for the 17 months of the war; the total casualties may have reached 35–40% 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 US 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 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 3.1 million km² (1.2 million mi2) 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".