The Mexican–American War, also known as the First American Intervention, the Mexican War, or the U.S.–Mexican 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 despite the 1836 Texas Revolution.

American forces invaded New Mexico, the California Republic, and parts of what is currently northern Mexico; meanwhile, the American Navy conducted a blockade, and took control of several garrisons on the Pacific coast of Alta California, but also further south in Baja California. Another American army captured Mexico City, and forced Mexico to agree to the cession of its northern territories to the U.S.

American territorial expansion to the Pacific coast was the goal of President James K. Polk, the leader of the Democratic Party. However, the war was highly controversial in the U.S., with the Whig Party and anti-slavery elements strongly opposed. Heavy American casualties and high monetary cost were also criticized. The major consequence of the war was the forced Mexican Cession of the territories of Alta California and New Mexico to the U.S. in exchange for $18 million. In addition, the United States forgave debt owed by the Mexican government to U.S. citizens. Mexico accepted the Rio Grande as its national border, and the loss of Texas. The political aftermath of the war raised the slavery issue in the U.S., leading to intense debates that pointed to civil war; the Compromise of 1850 provided a brief respite.

In Mexico, terminology for the war include (primera) intervención estadounidense en México (United States' (First) Intervention in Mexico), invasión estadounidense de México (The United States' Invasion of Mexico), and guerra del 47 (The War of 1847).

Background

Mexico was torn apart by bitter internal political battles that verged on civil war, even as it was united in refusing to recognize the independence of Texas. Mexico threatened war with the U.S. if it annexed Texas. Meanwhile President Polk's spirit of Manifest Destiny was focusing U.S. interest on westward expansion

Designs on California

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 John Tyler's administration suggested a tripartite pact that would settle the Oregon boundary dispute and provide for the cession of the port of San Francisco; Lord Aberdeen declined to participate but said Britain had no objection to U.S. territorial acquisition there.

For 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 proposal as expensive and a potential source of conflict.

Republic of Texas

The Republic of Texas. The present-day outlines of the U.S. states are superimposed on the boundaries of 1836–1845.

In 1810, 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 F. Austin, succeeded and brought over 300 families into Texas, which started the steady trend of American migration into the Texas frontier. Austin's colony was the most successful of several colonies authorized by the Mexican government. The Mexican government intended the anglophone settlers to act as a buffer between the existing Mexican residents and the marauding Comanches, but the anglo colonists tended to settle where there was decent farmland, rather than where they would have been an effective buffer. In 1829, as a result of the large influx of American immigrants, the Americans outnumbered Mexicans in the Texas territory. The Mexican government decided to bring back the property tax, increase tariffs on U.S. shipped goods, and prohibit slavery. The settlers rejected the demands, which led to Mexico closing Texas to additional immigration. However, Americans continued to flow into the Texas territory. In 1834, General Antonio López de Santa Anna became the dictator of Mexico, abandoning the federal system. He decided to squash the semi-independence of Texas. Stephen F. Austin called Texans to arms; they declared independence from Mexico in 1836, and after Santa Anna defeated the Texans at the Alamo (causing his army nearly three weeks' delay), he was defeated by the Texan Army commanded by General Sam Houston and captured at the Battle of San Jacinto and signed a treaty recognizing Texas independence. Texas consolidated its status as an independent republic, winning official recognition from Britain, France, and the U.S., which all advised Mexico not to try to reconquer the new nation. Most Texans wanted to join the U.S. but annexation of Texas was contentious in the U.S. Congress, where Whigs were largely opposed. In 1845 Texas agreed to the offer of annexation by the U.S. Congress. Texas became the 28th state on December 29, 1845.

The Mexican government had long warned the United States that annexation of Texas would mean war. Because the Mexican Congress had refused to recognize Texan independence, Mexico saw Texas as a rebellious territory that would be retaken. Britain and France, which recognized the independence of Texas, repeatedly tried to dissuade Mexico from declaring war. When Texas joined the U.S. as a state in 1845, the Mexican government broke diplomatic relations with the U.S.

The border of Texas as an independent state had never been settled. The Republic of Texas claimed land up to the Rio Grande based on the Treaties of Velasco, but Mexico refused to accept these as valid, claiming the border as the Nueces River. Reference to the Rio Grande boundary of Texas was omitted from the U.S. Congress' annexation resolution to help secure passage after the annexation treaty failed in the Senate. President Polk 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 if Mexico declared war while staying on good terms with the inhabitants. At the same time he wrote to Thomas Larkin, the American consul in Alta California, disclaiming American ambitions in California but offering to support independence from Mexico or voluntary accession to the U.S., 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 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. Frémont 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 area of California and visited Santa Cruz and the Salinas Valley, explaining he had been 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 many 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 ($632,500,000 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 ($76 million today) owed to U.S. citizens for damages caused by the Mexican War of Independence and pay another $25 to $30 million ($633 million to $759 million today) in exchange for the two territories.

Mexico was not inclined nor able 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 direct 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 to settle the problem of Texas annexation peacefully, 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 U.S.

Conflict over the Nueces Strip

President Polk ordered General Taylor and his forces south to the Rio Grande, entering the territory that Mexicans disputed. Mexico laid claim to the Nueces River—about 150 mi (240 km) north of the Rio Grande—as its border with Texas; the U.S. claimed it was the Rio Grande, citing the 1836 Treaties of Velasco. Mexico, however, under the leadership of General Lorenzo Chlamon, rejected the treaties and refused to negotiate; it claimed all of 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.

Declaration of war

Overview map of the war

Polk received word of the Thornton Affair, which, 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. Antonio Lopez de Santa Anna Once the U.S. declared war on Mexico, Antonio López 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 before. 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 U.S., 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 agreements. Santa Anna declared himself president again and unsuccessfully tried to fight off the U.S. invasion.

In the U.S., increasingly divided by sectional rivalry, the war was a partisan issue and an essential element in 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 slave-owning territory to the South and avoiding being outnumbered by the faster-growing North. John O'Sullivan, the editor of the "Democratic Review", 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 rise of a Slave Power; Whigs generally wanted to strengthen the economy with industrialization, not expand it with more land. Democrats wanted more land; northern Democrats were 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:

Northern abolitionists attacked the war as an attempt by slave-owners to strengthen the grip of slavery and thus ensure 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 primarily an effort to expand slavery in a speech he gave before the House on May 25, 1836. In response to such concerns, Democratic Congressman David Wilmot introduced the Wilmot Proviso, which aimed to prohibit slavery in new territory acquired from Mexico. Wilmot's proposal did not pass Congress, but it spurred further hostility between the factions.

Defense of the war

Besides alleging that the actions of Mexican military forces within the disputed boundary lands north of the Rio Grande constituted an attack on American soil, the war's advocates viewed the territories of New Mexico and California as only nominally Mexican possessions with very tenuous ties to Mexico, and as actually unsettled, ungoverned, and unprotected frontier lands, whose non-aboriginal population, where there was any at all, comprised a substantial—in places even a majority—American component, and which were feared to be under imminent threat of acquisition by America's rival on the continent, the British.

President Polk reprised these arguments in his Third Annual Message to Congress on December 7, 1847, in which he scrupulously detailed his administration's position on the origins of the conflict, the measures the U.S. had taken to avoid hostilities, and the justification for declaring war. He also elaborated upon the many outstanding financial claims by American citizens against Mexico and argued that, in view of the country's insolvency, the cession of some large portion of its northern territories was the only indemnity realistically available as compensation. This helped to rally Congressional Democrats to his side, ensuring passage of his war measures and bolstering support for the war in the U.S.

Opening hostilities

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 during the bombardment, and two were killed. Among the dead was Jacob Brown, after whom the fort was later named.

On May 8, Zachary Taylor and 2,400 troops arrived 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 fierce 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 unfamiliar 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 additional casualties as the withdrawing troops passed by the fort. Many Mexican soldiers drowned trying to swim across the Rio Grande.

Conduct of the war
After the declaration of war, U.S. forces invaded Mexican territory on two main fronts. The U.S. War Department sent a U.S. cavalry force under Stephen W. Kearny to invade western Mexico from Jefferson Barracks and Fort Leavenworth, reinforced by a Pacific fleet under John D. Sloat. This was done primarily because of concerns that Britain might also try to seize 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.

California campaign

Although the U.S. declared war against Mexico on May 13, 1846, it took over a month (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 maintain the peace between the States 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 slowly marching to Oregon when he received word that war between Mexico and the U.S. was imminent. So began his chapter of the war, the "Bear Flag Revolt."

A replica of the first "Bear Flag" now at El Presidio de Sonoma, or Sonoma Barracks

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 Monterey, the capital, on July 7 and raise the flag of the U.S.; San Francisco, then called Yerba Buena, was occupied 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. When Stockton's forces, sailing southward to San Diego, stopped in San Pedro, he sent 50 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 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 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.

Pacific Coast campaign

USS Independence assisted in the blockade of the Mexican Pacific coast, capturing the Mexican ship Correo and a launch on 16 May 1847. She supported the capture of Guaymas, Mexico, on 19 October 1847 and landed bluejackets and Marines to occupy Mazatlán, Mexico on 11 November 1847. After upper California was secure most of the Pacific Squadron proceeded down the California coast capturing all major Baja California cities and capturing or destroying nearly all Mexican vessels in the Gulf of California. Other ports, not on the peninsula, were taken as well. The objective of the Pacific Coast Campaign was to capture Mazatlan, a major supply base for Mexican forces. Numerous Mexican ships were also captured by this squadron with the USS Cyane given credit for 18 captures and numerous destroyed ships.[40] Entering the Gulf of California, Independence, Congress and Cyane seized La Paz captured and burned the small Mexican fleet at Guaymas. Within a month, they cleared the Gulf of hostile ships, destroying or capturing 30 vessels. Later on their sailors and marines captured the town of Mazatlan, Mexico, on 11 November 1847. A Mexican campaign under Manuel Pineda to retake the various captured ports resulted in several small clashes (Battle of Mulege, Battle of La Paz, Battle of San José del Cabo) and two sieges (Siege of La Paz, Siege of San José del Cabo) in which the Pacific Squadron ships provided artillery support. U.S. garrisons remained in control of the ports and following reinforcement, Lt. Col. Henry S. Burton marched out, rescued captured Americans, captured Pineda and on March 31, defeated and dispersed remaining Mexican forces at the Skirmish of Todos Santos, unaware the Treaty of Guadalupe Hidalgo had been signed in February 1848. When the American garrisons were evacuated following the treaty, many Mexicans that had been supporting the American cause and had thought Lower California would also be annexed like Upper California, were evacuated with them to Monterey.

Northeastern Mexico

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 U.S. 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 and repulsed Taylor's best infantry division at Fort Teneria. American soldiers, including many West Pointers, had never engaged in urban warfare before and they marched straight down the open streets, where they were annihilated by Mexican defenders well-hidden in Monterrey's thick adobe homes. Two days later, they changed their urban warfare tactics. Texan soldiers had fought in a Mexican city before and advised Taylor's generals that the Americans needed to "mouse hole" through the city's homes. In other words, they needed to punch holes in the side or roofs of the homes and fight hand to hand inside the structures. This method proved successful and Ampudia eventually surrendered.

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

Battle of Monterrey

Northwestern Mexico

On March 1, 1847, Alexander W. 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.

The civilian population of northern Mexico offered little resistance to the American invasion possibly because the country had already been devastated by Comanche and Apache Indian raids. Josiah Gregg, who was with the American army in northern Mexico, said that “the whole country from New Mexico to the borders of Durango is almost entirely depopulated. The haciendas and ranchos have been mostly abandoned, and the people chiefly confined to the towns and cities.”

U.S. press and popular war enthusiasm

During the war inventions such as the telegraph created new means of communication 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” realized the voracious need of the public to get the astounding war news. This was the first time in the American history when the accounts by journalists, instead of the opinions of politicians, caused great influence in shaping people’s minds and attitudes toward a war. News about the war always 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.

Desertion

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% (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 to join another U.S. unit and get a second enlistment bonus. While some deserted because of the miserable conditions in camp, it has been suggested that others used the army to get free transportation to California, where they deserted to join the gold rush. This, however, is unlikely as gold was only discovered in California on January 24, 1848, less than two weeks before the war concluded. By the time word reached the eastern U.S. that gold had been discovered, word also reached it that the war was over.

Battle of Churubusco by J. Cameron, published by Nathaniel Currier. Hand tinted lithograph, 1847. Digitally restored.

Scott's Mexico City campaign

Battle of Chapultepec

Landings and Siege of Vera Cruz 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. On March 9, 1847, Scott performed the first major amphibious landing in U.S. history 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 using specially designed landing craft. Included in the invading force were Robert E. Lee, George Meade, Ulysses S. Grant, James Longstreet, 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 the best 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: 

Pause at Puebla

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. During the following months Scott gathered supplies and reinforcements at Puebla and sent back units whose enlistments had expired. Scott also made strong efforts to keep his troops disciplined and treat the Mexican people under occupation justly, so as to prevent a popular rising against his army.

Advance on Mexico City and its capture

With guerrillas harassing his line of communications back to Vera Cruz, Scott decided not to weaken his army to defend it but, leaving only a garrison at Puebla to protect the sick and injured recovering there, advanced on Mexico City on August 7, with his remaining force. The capital was laid open in a series of battles around the right flank of the city defenses, culminating in the Battle of Chapultepec. With the subsequent storming of the city gates, the capital was occupied. Winfield Scott became an American national hero after his victories in this campaign of the Mexican–American War, and later became military governor of occupied Mexico City.

Santa Anna's last campaign

In late September 1847, Santa Anna made one last attempt to defeat the Americans, by cutting them off from the coast. General Joaquín Rea began the Siege of Puebla, soon joined by Santa Anna, but they failed to take it before the approach of a relief column from Vera Cruz under Brig. Gen. Joseph Lane prompted Santa Anna to stop him. Puebla was relieved by Gen. Lane October 12, 1847, following his defeat of Santa Anna at the Battle of Huamantla October 9, 1847. The battle was Santa Anna's last. Following the defeat, the new Mexican government led by Manuel de la Peña y Peña asked Santa Anna to turn over command of the army to General José Joaquín de Herrera.

Anti guerrilla campaign

Following his capture and securing of the capital, General Scott sent about a quarter of his strength to secure his line of communications to Vera Cruz from the Light Corps and other Mexican guerilla forces that had been harassing it since May. He strengthened the garrison of Puebla, and by November established 750 man posts at Perote, Puente Nacional, Rio Frio, and San Juan along the National Road and detailed an antiguerrilla brigade under Brig. Gen. Joseph Lane to carry the war to the Light Corps and other guerillas. He also ordered that convoys would travel with at least 1,300-man escorts. Despite some victories over General Joaquín Rea at Atlixco (18 October 1847) and Izucar de Matamoros (in November) by General Lane, guerrilla raids on the supply route continued into 1848 until the end of the war.

Treaty of Guadalupe Hidalgo

The Mexican Cession, shown in red, and the later Gadsden Purchase, shown in yellow.

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, New Mexico, most of Arizona and Colorado, and parts of Texas, Oklahoma, Kansas, and Wyoming. In return, Mexico received US $18,250,000 ($461,725,000 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 ($82,225,000 today) in debts that the Mexican government owed to U.S. citizens. The acquisition was a source of controversy then, 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 U.S., 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 control or 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

Mexican territorial claims relinquished in the Treaty of Guadalupe Hidalgo in white.

Article XI offered a potential benefit to Mexico in that the US pledged to suppress the Comanche and Apache raids that had ravaged northern Mexico. However, the Indian raids did not cease for several decades after the treaty, although a cholera epidemic reduced the numbers of the Comanche in 1849.

Results

American occupation of Mexico City

Mexican territory, prior to the secession of Texas, comprised almost 1,700,000 sq mi (4,400,000 km2), which was reduced to just under 800,000 by 1848. Another 32,000 were sold to the U.S. in the Gadsden Purchase of 1853, for a total reduction of more than 55%, or 900,000 square miles. The annexed territories, comparable in size to Western Europe, were essentially unsettled, containing about 1,000 Mexican families in Alta California and 7,000 in Nuevo México,[citation needed] as well as large Native American nations such as the Navajo, Hopi, and dozens of others. A few relocated further south in Mexico; the great majority remained in the U.S. 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

In much of the U.S., victory and the acquisition of new land brought a surge of patriotism. Victory seemed to fulfill Democrats' 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.

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, Robert E. Lee, and 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.

Grant's views on the war

President Ulysses S. Grant, who as a young army lieutenant had served in Mexico under General Taylor, recalled in his Memoirs, published in 1885, that:

    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.

Combatants

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.

United States

At the beginning of the war, the U.S. 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 looting and murder of Mexican civilians, mostly by State Volunteers. One officer's diary records: 

  John L. O'Sullivan, a vocal proponent of Manifest Destiny, later recollected:

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.

The last surviving U.S. veteran of the conflict, Owen Thomas Edgar, died on September 3, 1929, at age 98.

1,563 U.S. soldiers are buried in the Mexico City National Cemetery, which is maintained by the American Battle Monuments Commission.

Mexico

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. 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. The ease of the American landing at Vera Cruz was in large part due to civil warfare in Mexico City, which made any real defense of the port city impossible. As Gen. Santa Anna said, "However shameful it may be to admit this, we have brought this disgraceful tragedy upon ourselves through our interminable in-fighting."

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 ½ were hanged as deserters. The leader, John Riley, was merely branded since he had deserted prior to the start of the war.

Impact of the war in the U.S.

Despite initial objections from the Whigs and abolitionists, the war would nevertheless unite the U.S. 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 these, approximately 1.5% were killed in the fighting and nearly 10% died of disease; another 12% were wounded or discharged because of disease, or both. For years afterward, 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 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 at first hand were unenthusiastic. Anti-slavery elements opposed that position and fought for the exclusion of slavery from any territory absorbed by the U.S. 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 U.S. agreed to pay $15 million and assumed the claims of its citizens against Mexico. A final territorial adjustment between Mexico and the U.S. was made by the Gadsden Purchase in 1853.

"An Available Candidate: The One Qualification for a Whig President." Political cartoon about the 1848 presidential election which refers to Zachary Taylor or Winfield Scott, the two leading  contenders for the Whig Party nomination in the aftermath of the Mexican–American War. Published by Nathaniel Currier in 1848, digitally restored.

The war was one of the most decisive events for the U.S. in the first half of the 19th century. While it marked a significant waypoint for the nation as a growing military power, it also served as a milestone especially within the U.S. narrative of Manifest Destiny. The resultant territorial gains set in motion many of the defining trends in American 19th-century history, particularly for the American West. The war did not resolve the issue of slavery in the U.S. but rather in many ways inflamed it, as potential westward expansion of the institution took an increasingly central and heated theme in national debates preceding the American Civil War. Furthermore, in doing much to extend the nation from coast to coast, the Mexican–American War was one step in the massive migrations to the West of Americans, which culminated in transcontinental railroads and the Indian wars later in the same century.