The residents of Matamoros, Mexico, had been watching them for days. 100 yards across the Rio Del Norte, on its northern bank, General Zachary Taylor and his 3,550 troops—two army infantry regiments and the 2nd Dragoons—had been camped out since March 28th, 1846. President James Polk's orders in January of that year required that the general and his men march through the disputed territory between two rivers, the Nueces and the Del Norte, also known as the Rio Grande.
Ever since the United States had annexed Texas one year before, the chief executive considered all land north of the Rio Grande to be part of the new American state of Texas, and in effect, territory that the United States was bound to protect and defend. The Mexicans, already incensed at the Americans' annexation of their former territory, considered the border to be not the Rio Grande but rather the Rio Nueces, meaning that the land between the two rivers wasn't a part of Texas at all, but rather, part of the Mexican State of Tamaulipas.
And so, the Mexicans regarded any American military presence on that soil as an invasion of Mexican territory and an act of war. Anticipating this very disagreement, Polk's orders had been intended to provoke Mexico into starting a war that the president deeply desired.
For a month, while officers from both armies exchanged heated messages back and forth across the river, the women of Matamoros and the American soldiers exchanged flirtations. Encouraged by harsh conditions, low pay, widespread sickness, and the Mexicans themselves, multiple deserters from the U.S. Army fled across the river. Two dragoons, captured on the day of the American Army's arrival, were returned to the American ranks, brimming with stories for their comrades of their kind treatment from the Mexicans.
Despite these exchanges, tensions had escalated between the two armies. An American colonel had gone missing, and the patrol sent out after him was ambushed. Mexican General Pedro de Ampudia, having arrived in early April with 3,000 additional troops, had given General Taylor an ultimatum to withdraw back to the Nueces or face the commencement of hostilities. Taylor took Ampudia's message to mean the two nations were at war and blockaded the mouth of the Rio Grande on April 15th. The citizens of Matamoros, with 6,000 Mexican troops now in their midst and their supply route cut off by the Americans, were caught in the middle.
By the time General Mariano Arista had arrived to replace Ampudia, a skirmish between Mexican and American patrols had resulted in 16 American casualties, providing President Polk with the casus belli he needed to declare war. The Mexican-American War had begun.
When, in 1845, Congress authorized the annexation of Texas—a renegade Mexican province that had declared its independence in 1836—Mexico broke off diplomatic relations with the United States. But the Mexican government, weakened by a succession of revolutions and military coups since it won independence from Spain in 1821, soon recognized that diplomacy might be the only way to avoid war with its increasingly aggressive northern neighbor.
And Texas wasn't the only bone of contention; the Americans also wanted to take over Mexican lands in California and New Mexico, and private American citizens sought millions of dollars in repayments for losses they claimed to have suffered in Mexican business dealings due to the political chaos there.
In September 1845, the Mexicans agreed to negotiate with President Polk's envoy, a Louisiana politician named John Slidell, who offered the Mexicans a deal. The U.S. would assume all liability for the claims of American businessmen against Mexico if the Mexicans would cede to the United States the disputed territory between the Rio Grande and Nueces rivers. The Americans also wanted to buy New Mexico for $5 million and California for $25 million.
Slidell's arrival at the Mexican port of Vera Cruz set off a chain of events that helped to spark another coup. Rumors spread throughout Mexico that President José Joaquin Herrera was about to betray his country by giving away Mexican territory to the Americans. Hoping to protect himself against these charges of treason, Herrera abruptly decided to refuse to meet with Slidell. But it was too late. Major General Mariano Paredes won support from the military and the church to overthrow the Herrera government, promising to resist American encroachments against Mexican interests.
Mexico's refusal to receive Slidell and pursue a negotiated settlement made it plausible for President Polk to claim that he'd sought a peaceful solution to America's problems with Mexico, only to be rebuffed and forced to turn to a military solution as a last resort. In fact, Polk deeply desired war with Mexico and the military takeover there helped him get his war. Slidell's rejection led directly to the president's order to General Zachary Taylor to march into the disputed lands south of the Nueces, which in turn led directly to the opening of armed combat.
16 American soldiers became casualties in the first skirmish, giving Polk the justification he'd been looking for to seek funding from Congress for a war it had never formally authorized.
Only 14 representatives and two senators voted against the spending bill. Many of those who did vote for the measure were skeptical of the administration's version of the events that led to the conflict, but for political reasons—especially since the fighting had already begun and without funding the soldiers would be stranded—they didn't withhold the war appropriations. While the conflict had a few outspoken and even famous critics, most Americans enthusiastically supported it.
General Santa Anna of Mexico seized power in the early days of the war and mounted a fight that lasted almost two years. But by the end of 1847, General Zachary Taylor had become a national hero in the United States for winning two victories at the Rio Grande and mounting a successful advance into northeastern Mexico, where he took Monterrey and routed Santa Anna's army at the battle of Buena Vista.
General Winfield Scott invaded central Mexico via the port of Vera Cruz and—for the first time in American history—U.S. forces took the capital of a foreign nation. Diplomat Nicholas P. Trist negotiated a settlement (which the defeated Mexicans had little choice but to accept) whereby the United States paid $15 million for territory that included Texas (with a southern boundary along the Rio Grande) and most of what later became the American southwest.
Mexico lost over half of its territory.
Writer, philosopher, author, naturalist, and activist Henry David Thoreau called the Mexican-American War "the work of comparatively a few individuals using the standing government as their tool." He argued that "in the outset, the people would not have consented to this measure."blank">Civil War.
Manifest Destiny ultimately collided with the country's greatest contradiction: the thorny problem of how to decide whether or not slavery would be allowed in the vast new territories acquired from Mexico would prove to be the undoing of the Union in 1861.
Philosopher, lecturer, and intellectual Ralph Waldo Emerson proved prophetic when he wrote that America's swallowing up of over half of Mexico would be like a man who swallows arsenic, "which brings him down in turn. Mexico will poison us."
James Knox Polk was a loyal Jackson Democrat, playing a central role in the Bank War of 1832 to 1836 before becoming the main supporter of the Jackson administration while serving as Speaker of the House of Representatives in Congress. Polk was a slaveholder from Tennessee, where he became governor in 1839; he fashioned himself after Jackson and was an ardent proponent of annexation as a means of expanding the territorial holdings of the United States.
In 1844, the Democratic Party was set to nominate former president Martin Van Buren, himself a savvy politician who'd dedicated most of his long career to avoiding a showdown between the North and the South over the issue of slavery. However, when Van Buren came out against a popular push to annex Texas in order to avoid conflict over the slavery issue, he ended up sparking dissension within the ranks of the party.
Democratic delegates who supported expansion, including most party members from the South, maneuvered to block Van Buren and instead nominated the pro-expansion Polk as the first so-called "dark-horse"—or underdog—candidate for the presidency.
Polk managed to win over the various feuding factions of Democrats by promising—among other things—that he'd only serve for one term and by coming out in support of the Tariff of 1842, despite his anti-protectionist record. Not surprisingly, though, Polk later reneged on the tariff issue.
Polk and the Democrats heralded the promise and benefits of territorial expansion above all other matters, hoping that a focus upon the West would drown out the threat of sectional discord between North and South.
Democrats referred to the "clear and unquestionable" title that America held over the Oregon Territory (which Britain also claimed to own) and Texas (which had declared itself an independent republic and had been warring with Mexico). In response, Whigs—the main opposition party at the time—taunted Polk for his lack of political stature, particularly in contrast to their own nationally renowned candidate, Senator Henry Clay of Kentucky.
Clay tried to appease Southern Whigs by saying that he had no personal objection to the annexation of Texas, but that he thought it could destroy the country. In a telling indication of the growing sectional divide, especially among Whigs, Clay vacillated on the annexation issue in a futile attempt to appeal to both the Northern and Southern contingents of his party.
In the end, Polk barely eked out a victory. He lost his own home state of Tennessee but won the national vote by the tiny margin of 1.4%, a result made possible in part by the massive turnout of Irish Democrats in New York.
Polk's successful expansionist policies created unexpectedly divisive new political conflicts. In August 1846, an obscure congressman from Pennsylvania, Democrat David Wilmot, tacked a proviso onto an appropriations bill that President Polk had sent to the House.
The proviso borrowed language from Thomas Jefferson's Northwest Ordinance, which said that "neither slavery nor involuntary servitude shall ever exist in any part of said territory, except for crime, whereof the party shall first be duly convicted." Wilmot sought to apply this provision to any new territory acquired through the Mexican-American War, blocking the expansion of slavery into the West. In so doing, Wilmot enraged the South, split the Democratic majority along sectional lines, and infuriated President Polk, who wanted to avoid the slavery issue altogether while obtaining as much territory as possible.
Though ultimately defeated in the Senate, the Wilmot Proviso would be re-introduced by the Whig majority in the House during each session of Congress for the next four years, provoking a heated sectional debate that many historians have recognized, in retrospect, as a pivotal precursor to the Civil War.
At the time it was first proposed, the Wilmot Proviso offered a means by which Northern Whigs could appear supportive of national troops while withholding their endorsement of the Mexican-American War. Many Northern Democrats found the proviso appealing because it would enable them to embrace the popular concept of free labor in the process of acquiring any conquered territories. Many white voters envisioned free labor as a means of earning a decent and respectable wage in the emerging capitalist system, without the presence, interference, or competition that slavery represented.
The Mexican War seemed to reinforce partisan allegiances on both sides of the political spectrum, for Democrats defended the conflict as justifiable while most Whigs railed against it. Yet this war sparked issues that continually threatened to undermine the national parties by realigning their members according to their sectional interests. At the time, a sufficient number of Southern senators could join together to defeat the Proviso, uniting pro-annexationist Democrats (mostly Southerners) with the anti-annexationist Whigs (mostly Northerners).
The Southern contingent of both parties joined forces to fight any attempt to block slavery's expansion into new states. Northerners, in turn, resented the institution of slavery and the three-fifths compromise that enabled Southern congressmen to exert an influence disproportionate to the actual number of voters in their respective constituencies. Because of the three-fifths compromise, the slave population bolstered the voting power of Southern whites.
As historian Charles Sellers has argued, "what Wilmot called the 'White Man's Proviso' signaled a fusion of antislavery with racism that proved unstoppable."
In other words, politicians who claimed to stand for "free soil, free labor, and free men" could derive political popularity by standing against the expansion of slavery on behalf of white men.blank">Abolitionists—motivated by a sincere desire to help enslaved Blacks—remained a small minority in the North, but working-class whites who cared little for the fate of Black people increasingly came to oppose the institution of slavery because they viewed it as a competitive threat to their own jobs and an institution that undercut and demeaned the value and honor of free labor.