How the U.S.-Mexico War reshaped America

Following its independence from Spain in 1821, Mexico was navigating the challenges of its new-found freedom, grappling with internal divisions and a fragile political structure.

The United States, having secured its independence several decades earlier, was experiencing a period of rapid growth and expansion.

The doctrine of Manifest Destiny, a term coined by journalist John L. O'Sullivan in 1845, encapsulated the prevailing American belief that the United States was destined to expand across the North American continent.

The United States was experiencing a period of economic growth and prosperity, which fueled the desire for additional land for agriculture, trade, and settlement.

Mexico's economy, on the other hand, was struggling, and the country was politically unstable after achieving independence, making it difficult to effectively govern and protect its northern territories.

The centralist policies of General Antonio López de Santa Anna had provoked the federalist and liberal factions within the country, leading to instability and unrest. 

After winning its independence from Mexico in 1836, Texas functioned as an independent republic.

Mexico, however, considered Texas a breakaway state and refused to recognize its sovereignty.

When the U.S. Congress voted to annex Texas on February 28, 1845, and Texas was formally admitted to the Union on December 29, 1845, Mexico severed diplomatic relations with the United States.

The annexation heightened tensions over the undefined border between Texas and Mexico.

The United States claimed the Rio Grande as the border, based on the Treaties of Velasco signed by Santa Anna after his capture during the Texas Revolution.

Mexico, however, claimed that the border lay farther north along the Nueces River.

President Polk, a proponent of Manifest Destiny, also had broader ambitions beyond the Texas issue.

He eyed the Mexican territories of California and New Mexico and had made an unsuccessful attempt to purchase them from Mexico.

Polk's expansionist agenda was clear, and the disputed Texas border provided a convenient pretext for advancing American interests further into Mexican territory.

In January 1846, President James K. Polk ordered General Zachary Taylor to move his troops into the disputed Nueces Strip, and by March, Taylor had established Fort Texas (later Fort Brown) near the present-day city of Brownsville.

The conflict was precipitated on April 25, 1846, when a detachment of U.S. troops was attacked by Mexican forces in the disputed zone, resulting in the Thornton Affair.

Polk, asserting that American blood had been shed on American soil, requested a declaration of war from Congress, which was issued on May 13, 1846.

Mexico, which had declared that annexation of Texas by the U.S. would be a cause for war, responded by formally declaring war on July 7, 1846.

The first major engagement was the Battle of Palo Alto on May 8, 1846, where U.S. forces led by General Zachary Taylor repelled a Mexican attack, utilizing superior artillery to their advantage.

The following day, the U.S. forces engaged the Mexican army again at the Battle of Resaca de la Palma, achieving a decisive victory and forcing the Mexican troops to retreat across the Rio Grande.

General Taylor then led his troops across the river to occupy the city of Matamoros, and by late September 1846, he had moved further into Mexican territory, capturing the city of Monterrey after a hard-fought battle.

The Battle of Monterrey, from September 21 to 24, was a significant victory for the U.S. forces, but it also highlighted the war's potential costs, leading to a brief armistice between the two armies.

Meanwhile, a second U.S. expedition under the command of Colonel Stephen W. Kearny was making its way westward to secure the territories of New Mexico and California.

Kearny's troops captured Santa Fe without a fight on August 18, 1846, and he established a provisional government in New Mexico.

In California, American settlers had already staged the Bear Flag Revolt in June 1846 and declared the Republic of California, with U.S. forces arriving shortly after to take control.

The focus of the war then shifted to the campaign to capture Mexico City. In March 1847, General Winfield Scott landed near the port city of Veracruz, which he besieged and captured after a 20-day bombardment, beginning on March 9.

This victory gave the U.S. a critical foothold on the Mexican coast and opened the way for an advance toward the capital.

The most audacious campaign of the war was Scott's march inland toward Mexico City.

The U.S. forces faced rugged terrain, disease, and strong Mexican resistance. Key victories at the Battle of Cerro Gordo in April, the skirmishes near Puebla in May, and the twin battles of Contreras and Churubusco in August paved the way for the final assault on the Mexican capital.

The climactic moment of the campaign came at the Battle of Chapultepec on September 13, 1847.

Young military cadets, known as Los Niños Héroes, became symbols of Mexican resistance in this battle, fighting to the death defending the castle of Chapultepec.

Despite their valor, the fortress fell to the U.S. forces, which cleared the path to Mexico City.

The American army entered the capital on September 14, 1847, effectively ending the war's major combat operations.

In the United States, the war was a subject of intense political debate. Supporters of President Polk and the Democratic Party generally favored the war, viewing it as a divine opportunity to spread the republic's democratic ideals and fulfill the country's Manifest Destiny.

This sentiment was particularly strong among Southerners who saw the potential for expansion as a means to extend slavery and bolster their economic interests.

However, there was significant opposition to the war, particularly from the Whig Party and notable figures such as John Quincy Adams and Henry Clay.

The war's critics accused Polk of provoking an unnecessary conflict and viewed the expansion as an imperialist grab for more slave territory.

Abraham Lincoln, then a young congressman, challenged Polk's justification for the war with his "Spot Resolutions," demanding to know the precise "spot" on American soil where blood had first been shed.

In the United States, the war was distant for most civilians, but it still touched the lives of many, particularly those with family members serving in the conflict. 

Public opinion was also shaped by the media of the day, with newspapers playing a crucial role in swaying or reflecting public sentiment.

The advent of the telegraph and more rapid dissemination of news allowed for more immediate coverage of the war, which both informed and inflamed public opinion.

For American soldiers, the war was an arduous campaign, marked by long marches over difficult terrain, diseases like dysentery and yellow fever, and the challenges of fighting in an unfamiliar land.

Many were volunteers, driven by a sense of adventure or patriotic fervor, but they often found themselves unprepared for the harsh realities of war.

The professional soldiers of the regular army, better trained and disciplined, bore the brunt of the fighting and often viewed the volunteers with a mix of disdain and envy for their lack of discipline but also their enthusiasm.

Mexican soldiers, on the other hand, faced not only the invading forces but also the internal turmoil of a nation with a fragmented political landscape.

They were often poorly equipped and underpaid, if paid at all, which led to issues with morale.

Despite these challenges, many Mexican soldiers fought tenaciously, driven by a fierce determination to defend their homeland from what they viewed as an act of aggression by a more powerful neighbor.

It is important to note that the civilians in Mexico bore the brunt of the war's devastation.

Towns and cities were turned into battlegrounds, with the sieges and battles causing significant damage to property and infrastructure.

The American blockade of Mexican ports and the subsequent invasion disrupted trade, leading to economic hardship and shortages of basic goods.

The movement of armies across the Mexican countryside also took its toll on agriculture, leading to food shortages and contributing to the suffering of the local population.

The fall of Mexico City in September 1847 had left the Mexican government with little choice but to seek terms for peace.

Nicholas Trist, the chief clerk of the State Department, had been dispatched by President Polk to negotiate with the Mexican authorities.

Despite being recalled by Polk due to a lack of progress and his own overstepping of instructions, Trist decided to stay in Mexico and continue the negotiations, believing it was in the best interest of both nations to end the war swiftly.

The Mexican government was under pressure from a populace that felt betrayed and a military still reeling from defeat.

Nonetheless, the dire situation forced Mexico to cede a vast portion of its territory, including all of present-day California, Nevada, Utah, most of Arizona, and parts of New Mexico, Colorado, and Wyoming.

In return, the United States agreed to pay $15 million to Mexico and to assume up to $3.25 million in debts owed by Mexico to American citizens.

The Treaty of Guadalupe Hidalgo, signed on February 2, 1848. extended the United States' territory from the Atlantic to the Pacific.

The treaty also laid out terms for the protection of the rights of Mexican citizens living in the newly acquired territories, although the implementation of these protections was inconsistent and often subject to local interpretation.

In Mexico, the treaty was met with a mix of relief and resentment. The loss of over half of the nation's territory was a bitter pill to swallow, but the end of the war brought an end to the devastation and occupation.

The Mexican Congress ratified the treaty on May 19, 1848, after much debate and amidst a climate of national mourning for the lost territories.

For the United States, the treaty's ratification, which followed on March 10, 1848, after Senate amendments, was a cause for celebration for some and a source of concern for others who feared the expansion of slave territory.

The acquisition of new lands would soon lead to intense national debates over the extension of slavery, which would contribute to the sectional conflicts that eventually led to the Civil War.