Texas lay at the root of the conflict between Mexico and the United States. The area had long been coveted by Anglo-Americans as they pushed westward across the continent. During the early nineteenth century, American adventurers periodically led privately financed "filibuster" expeditions into Texas with the goal of wresting the region away from Spanish control. When Mexico won its independence in 1821, it inherited from Spain a deep distrust of American territorial ambitions. The United States lost little time in confirming these fears. Hoping to take advantage of the new nation’s severe financial difficulties in the wake of its decade-long revolutionary struggle, two presidents— John Quincy Adams and Andrew Jackson—offered to buy Texas, only to be firmly rebuffed by Mexican nationalists committed to maintaining sovereignty over the region.
In 1834, in an effort to consolidate his political power, Mexican president Antonio López de Santa Anna revoked the federalist Constitution of 1824. The move ignited a wave of rebellions and resistance throughout the country. Above the Rio Grande, Tejanos and Anglo-Texans initially joined in common cause against Santa Anna’s centralist rule. But the Texas rebellion soon proved entirely different than uprisings in other provinces, as thousands of Anglo-American volunteers poured across the Sabine River, lured by the prospect of adventure and land bounties. Texas declared its independence in March, 1836, and the following month routed Santa Anna’s army at the Battle of San Jacinto.
Anglo-Texans regarded independence as a temporary measure only, and the new Texas republic promptly applied for admission to the United States. A small clique of northern antislavery congressmen—-led by John Quincy Adams, now a staunch opponent of expansion—-blocked the initiative in 1839. The annexation issue was shelved for the next several years, but for southern leaders Texas remained a coveted prize, upon which their hopes for the expansion of the slave empire depended. Mexico, however, refused to recognize Texas’s independence, maintaining instead the position that its former province was in a state of rebellion. Twice in 1842 the Mexican government dispatched troops to seize San Antonio, announcing that these forays were the vanguard of a full-scale invasion. But the campaign to regain Texas never occurred, and the following year Texas and U.S. diplomats reopened annexation talks.
Although the U.S. Senate voted down an annexation treaty in 1844, the measure passed by a narrow margin when resubmitted by President Tyler as a joint resolution. Tyler signed the bill, known as the Brown Resolution, as his last act in office on March, 4, 1845. Two days later, Mexico’s minister in Washington, Juan N. Almonte, asked for his passports and sailed for home, severing diplomatic relations between his country and the United States.
At this point, the Mexican government had few options left to prevent the impending marriage between the United States and Texas. Abandoning the position that Texas was still a province in revolt, the government of José Joaquín de Herrera reluctantly offered to recognize the Republic as a sovereign nation—but only if it would promise to reject annexation to the United States. Texas president Anson Jones agreed to present the idea to the voters of Texas, leaving open the possibility in the early summer of 1845 that the Republic might reject the U.S. annexation offer.
The Mexican proposal stood little if any chance of success. Support for a union with the United States had always been strong in Texas, and had gained momentum in recent months as an ever-increasing tide of Americans poured into the region. Be that as it may, the new Polk administration greeted news of the offer with considerable alarm. Adding to these concerns were reports that Mexico, urged on by the British, planned to attack Texas should annexation take place. Such fears, too, had little basis in fact. Mexico lacked the resources to launch a military campaign, while Her Majesty’s government had warned its leaders that they could expect no aid from Great Britain should they attempt to do so.
An avowed expansionist, James K. Polk was determined to thwart any last minute efforts by Mexico to block annexation. Washington prevailed upon Texas to accept U.S. military protection, and on June 15 1845 the War Department ordered General Zachary Taylor, commander of a force of 1500 men stationed in western Louisiana, to march to the Texas frontier. On the same day, Polk took the added precaution of ordering an American naval build-up in the Gulf of Mexico. On July 4, a special convention voted to join the United States under the terms provided by the Brown resolution, dealing a final blow to Mexico’s anti-annexation campaign. When Taylor and his troops arrived in Texas by land and sea routes at the end of July, the War Department instructed them to take up positions at Corpus Christi, on the south bank of the Nueces River.
With the decision to station U.S. troops below the Nueces, the crisis between the United States and Mexico entered a new and critical phase. Mexico and Texas both claimed the land between the Nueces and the Rio Grande. The Nueces had served as the boundary of Texas under Spanish and Mexican rule; even as late as 1835, Anglo-American settlers recognized the Nueces, not the Rio Grande, as the legitimate boundary of Texas. Texas’s claim to the disputed area rested largely on the fact that Santa Anna, as a prisoner after the battle of San Jacinto, had accepted the Rio Grande boundary in a secret provision of the so-called Treaty of Velasco ending the Texas revolt. However, the treaty had never been considered—much less ratified—by the Mexican government. As a result, Texas unilaterally claimed the trans-Nueces in the decade that followed, but had made virtually no effort to extend its jurisdiction over the territory. Sparsely populated, the trans-Nueces had suffered in recent years as a result of increasingly destructive Comanche raids. Nonetheless, the small Mexican population that remained—there were no Anglos—was still governed by Mexican officials and subject to Mexican laws.
In the United States, it was generally assumed that while Mexico no longer had any claim to Texas, Washington would have to pay compensation to secure its recognition of the Rio Grande as the boundary. As a presidential candidate a year earlier, Polk had favored compensation for Mexico. Even the Brown resolution recently passed by the U.S. Congress implicitly acknowledged the validity of the Mexican claim, stipulating that "all questions of boundary" would be adjusted later.
But in the summer of 1845 the Polk administration aimed to send a clear and unequivocal message to Mexico--and its principal ally, Great Britain--that it would brook no interference in its plans to annex Texas. It further wished to demonstrate to any wavering Texans who might still oppose annexation that it could rely upon the United States to defend their interests. Thus, the administration’s decision to order Taylor into the trans-Nueces marked a complete departure from the position taken by Polk and the U.S. Congress only a few months earlier. More importantly, it was also a position that Mexican conservatives regarded as grounds for war.
Throughout the summer, Washington remained vigilant, responding periodically to new rumors of Mexican troop activity below the Rio Grande. On July 30, Secretary of War Marcy ordered General Taylor to "approach as near the boundary line, the Rio Grande, as prudence will dictate." Waiting for reinforcements, Taylor did not break camp at Corpus Christi until the following spring. With his army numbering almost 4000, Taylor marched south in March, 1846. Directly across the river from Matamoros, U.S. troops erected Fort Texas (later renamed Fort Brown, the site of modern day Brownsville). From this vantage point, U.S. artillery could fire into the plaza of the Mexican town. Along the coast nearby, U.S. gunships blockaded the mouth of the Rio Grande.
A clash between the two opposing armies had now become inevitable. On April 25, 1846, Mexican troops crossed the river near Fort Texas and attacked a squadron of U.S. dragoons. The engagement left sixteen U.S. troops killed or wounded, with the remaining fifty-two captured. News of the incident reached Washington D.C. two weeks later. On May 11 President Polk asked Congress for a declaration of war, maintaining that a state of war between the two nations already existed. He declared: "Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon the American soil."