How the U.S.-Mexico War reshaped America

In 1845, the annexation of Texas caused a serious disagreement between governments between the United States and Mexico.

Mexico had gained its independence from Spain in 1821 and inherited a northern frontier that included the provinces of New Mexico, Texas, and Alta California.

During the early 19th century, Mexican officials encouraged American settlers to migrate to Texas under strict conditions: they were expected to adopt Catholicism and pledge loyalty to Mexico, yet many resisted or only superficially complied with either requirement.

By the early 1830s, around 30,000 American settlers had entered Texas, outnumbering the local Tejano population and intensifying cultural and political friction.

As a result, tensions escalated and erupted into open conflict. 

After Texas declared independence in 1836, a brief but violent confrontation broke out with Mexican forces.

The siege of the Alamo ended in massacre, although the Texan victory at San Jacinto secured their de facto independence.

However, the United States delayed annexation for nearly a decade, aware that it would provoke war with Mexico and inflame disputes between North and South.

President James K. Polk supported annexation and admitted Texas as the twenty-eighth state, claiming that the Rio Grande was its southern border.

Mexican authorities maintained that the true boundary remained at the Nueces River, which lay far to the north. 

In an effort to avoid war while still expanding U.S. territory, Polk sent a team of negotiators, led by John Slidell, who attempted to buy California and New Mexico.

Mexican leaders refused to negotiate. Instead of withdrawing, Polk ordered General Zachary Taylor to place his troops in the area both sides claimed.

On 25 April 1846, Mexican cavalry attacked an American patrol, killing or wounding sixteen soldiers.

Polk used the incident to justify war, claiming that American blood had been spilled on American soil, although the skirmish occurred in territory that Mexico considered its own.

Congress declared war on Mexico on 13 May. 

From the outset, the United States launched military operations on multiple fronts.

In northern Mexico, General Zachary Taylor commanded the first major engagements when his army defeated Mexican forces at the Battles of Palo Alto and Resaca de la Palma in May 1846.

After crossing the Rio Grande, Taylor captured Monterrey after several days of street fighting, during which his forces fought troops led by General Pedro de Ampudia.

In February 1847, his outnumbered troops held their position at Buena Vista, repelling Santa Anna’s larger army and securing a key American victory.

In that battle, approximately 5,000 American soldiers resisted a force of about 15,000 Mexicans. 

In the southwestern theatre, Colonel Stephen W. Kearny advanced into New Mexico and occupied Santa Fe without resistance.

From there, he moved west to join naval forces and American settlers who had launched the Bear Flag Revolt in California.

Among the leaders of this revolt was William B. Ide, who briefly served as president of the short-lived California Republic in June 1846.

After several small clashes, U.S. forces secured control of the region. Naval commanders such as Commodore John D. Sloat and Commodore Robert Stockton coordinated closely with land forces to take key coastal positions, such as the ports of Monterey and San Francisco. 

In March 1847, General Winfield Scott opened a new front with an combined land and sea attack at Veracruz, with a force of around 12,000 troops who landed along the coast in what became the largest land-and-sea military action in American history until World War II.

After a successful siege, Scott began a campaign inland toward Mexico City. His army, though far from its supply base, advanced steadily and defeated Mexican troops at Cerro Gordo, Contreras, and Churubusco.

Following those victories, Scott's forces fought in difficult battles at Molino del Rey.

On 13 September, U.S. troops assaulted Chapultepec Castle, an important military stronghold.

According to Mexican accounts, six military cadets known as the Niños Héroes died defending the fortress and later became famous for their role in defending the country.

The next day, American soldiers entered Mexico City and captured the capital, which brought the fighting to a close. 

Among political leaders, the war exposed serious disagreements over the purpose and morality of American expansion.

Southern Democrats welcomed the conflict, since they saw it as a chance to acquire land suitable for cotton cultivation and the extension of slavery.

They praised Polk for standing firm against Mexican resistance and celebrated each battlefield success as evidence of national greatness. 

In the North, opposition grew rapidly. Many Whigs criticised the war as a war of conquest, engineered to expand slave territory.

Abraham Lincoln, then serving in the House of Representatives, introduced the “Spot Resolutions,” by insisting that Polk identify the exact location of the initial skirmish.

His criticism questioned the president’s honesty and cast doubt on the fairness of the conflict.

Though the resolutions failed to gain support, they gave voice to growing discontent.

Other critics, such as Henry David Thoreau, refused to support the war financially and instead explained their dissent in essays such as "Civil Disobedience." 

In August 1846, Representative David Wilmot proposed an amendment to ban slavery in any territory gained from Mexico.

The Wilmot Proviso passed in the House but failed in the Senate, revealing the depth of conflict between regions.

Introduced as part of a military funding bill, the proposal provoked heated debate.

Supporters in the North argued that slavery must be contained to preserve free labour and prevent the spread of Southern control over national decisions.

Southern leaders insisted that any restriction violated property rights and threatened the constitutional balance. 

During the war, arguments among lawmakers broke apart existing alliances. The Whig Party struggled to find a consistent position, as some members opposed the war on moral grounds while others supported the troops without endorsing Polk’s strategy.

Meanwhile, new political movements emerged in the wake of the conflict. For example, the Free Soil Party attracted Northern voters who rejected the expansion of slavery and still supported westward growth.

Although the United States won on the battlefield, the consequences of the war deepened sectional hostility and sowed seeds of future conflict. 

On 2 February 1848, the Treaty of Guadalupe Hidalgo officially ended the war.

Under its terms, Mexico accepted the Rio Grande as the boundary of Texas and ceded a huge area of land to the United States.

The territory included present-day California, Nevada, Utah, most of Arizona, and portions of New Mexico, Colorado, and Wyoming.

This transfer amounted to approximately 529,000 square miles: over half of Mexico’s national territory.

In return, the United States agreed to pay Mexico $15 million and to cover $3.25 million in claims made by American citizens. 

For Mexico, the loss proved damaging, since it damaged its economy and government structure, and widened the rifts between competing factions.

Mexican citizens in the ceded territory were granted the option to become U.S. citizens and were promised protection of their property, although enforcement of these protections was often inconsistent and weak.

In practice, many experienced legal discrimination, forced land sales, and exclusion from involvement in government decisions.

Under the California Land Act of 1851, landowners were required to prove title in English-language courts, which caused many Mexican landowners to lose their land grants. 

In the United States, the acquisition of the Mexican Cession fulfilled Polk’s territorial ambitions.

Settlement sped up rapidly, particularly after the discovery of gold in California in 1848.

Congress faced renewed disputes as it considered how to organise the territories.

These debates ended in the Compromise of 1850, which admitted California as a free state, put in place a stricter Fugitive Slave Law, and postponed further confrontation.

Compromises delayed conflict but did not resolve the fundamental disagreements. 

Among the generation of officers who served in Mexico, many would later command armies during the Civil War.

Ulysses S. Grant, Robert E. Lee, Thomas “Stonewall” Jackson, George B. McClellan, and Jefferson Davis all gained field experience during the campaign.

Davis, who served under Taylor, also advocated for the adoption of the Mississippi rifle.

The war also influenced public attitudes toward how Americans viewed their military, the meaning of support for the country, and how far a president's control could extend.

Although the United States expanded its territory and improved how it was seen overseas, yet the war created internal division that made the country more divided than before.