The War of Manifest Destiny: The Origins of the Mexican War (Apr 6, 2025)

by Scott Sosebee

Depending on how you count and what you consider as one, the United States has engaged in fifteen wars in its almost 250 years of existence. Wars are never anything to be glorified, and their origins and outcomes are never as simple and cut-and-dried as many try to pretend. The nation’s leaders—understandably, I guess, in a country that operates as a representative republic democratically elected—have tried to justify each and every one of those conflicts as morally defensible and often necessary. Such a defense is easier for some than others; World War II, fought to stop an authoritarian dictator from spreading his power to most of the world, falls into such a “positive” category. The campaigns to rid Native Americans of land they had occupied for centuries and end any semblance of self-government for them as well are much less unassailable. Another war that I would say falls into, at best, an ambiguous area of defense is the war the United States fought with Mexico from 1846-1848. It was a campaign designed for one purpose—to take territory from another nation, one we can rightfully call a “war of Manifest Destiny.”

The United States declared war on Mexico on May 13, 1846, but the origins of that war stretch back further and are related to the annexation of Texas, although it is more accurate to say that President James K. Polk used the excuse of a disputed border between Texas and Mexico to begin a war that he hoped would bring him the prize he so desperately wanted: the natural harbors of California that the United States could use to enter the Pacific Ocean trade and complete its desire to be a trans-continental nation.

When the Texian forces under Sam Houston defeated a Mexican force under Mexican President/General José Antonio López de Santa Anna at San Jacinto on April 21, 1836, to secure Texas’ independence, the new nation of Texas claimed that its border with Mexico was the Río Grande, from its mouth to the source. Such a declaration was dubious as first Spain and later Mexico had always contended that the border between their provinces was the Nueces, not the more southern river. Texas’ new leaders did so because claiming the Río Grade “from its mouth to its source” allowed it to also claim Santa Fe and the bulk of what was New Mexico. Also, it would include the vast pasture lands of South Texas. Mexico, of course, not only did not recognize Texas’ border claim but never acknowledged their former province’s independence and further declared that Texas was still its sovereign soil. Furthermore, Mexico was adamant that any attempts by the United States to annex Texas would be considered an act of war. Texas and Mexico uneasily coexisted on that tenuous border for the nine years of the Republic of Texas, and the United States refrained from any actions to make Texas part of its territory.

The situation began to change with the Presidential Election of 1844. Democratic nominee James K. Polk made annexing Texas one of his most adamant campaign promises, and when he was elected in November 1844, sitting President John Tyler and the Democratic majority Congress put the process in motion, one that would eventually result in Texas becoming part of the United States on December 28, 1845. Polk and his Administration were well aware of Mexico’s policy and that annexing Texas very well could begin a war with Mexico. Not only was he aware, but Polk was likely hoping it would lead to war, which a United States victory would allow it to dictate terms that would include taking California and a large swath of Mexican territory in the middle of the North American continent.

Even before Texas’ annexation was complete, Polk had sent John Slidell to Mexico City in November 1845 to begin a dialogue with the Mexican authorities. The negotiations were not cordial. Mexico believed that Slidell was there to at least negotiate about the disputed border and perhaps to find accommodation about annexing Texas. Slidell, somewhat impudently, informed his Mexican counterparts that the Texas annexation was fait accompli and that the border of the Río Grande was also not under consideration. Instead, he offered to buy Alta California and New Mexico for $25 million. Insulted, Mexico refused to meet with Slidell and broke diplomatic relations—probably the exact result that Polk wanted.

Polk then ordered General Zachary Taylor to move from Fort Jessup in Louisiana with 4,000 men in March 1846 to the Río Grande. Taylor made his way through Texas in good time, arriving at the river in mid-April 1846, where he made camp (which would become the origins of Brownsville). At the same time, Mexico ordered General Mariano Arista to move to the south bank of the river to oppose any American attempts to invade. The two armies were now faced off against each other. Taylor moved even further to antagonize his Mexican opponents. He moved the bulk of his artillery to a position that directly threatened the Mexican city of Matamoros. The United States Navy then arrived and blockaded the city from any relief that may come from the Gulf of Mexico. For two weeks, Arista held his forces and made no attempts to break the American blockade as he waited for orders and perhaps a peaceful solution, but his soldiers were cut off, and if he could not break the blockade, they faced dire consequences. Finally, on April 24, Arista ordered a force to cross the Río Grande and counter the American move. Taylor moved a small force to oppose them, and the Mexican army opened fire, killing eleven American soldiers. Taylor would eventually counter and, in what has become known as the Battle of Palo Alto, drove the Mexican army from the field. The two armies would later fight a bigger engagement at Resaca De La Palma.

Polk now had his war message. On May 13, he informed Congress that Mexico had invaded and “shed American blood on American soil.” Many northern Americans had been against war with Mexico, especially those belonging to the Whig Party. But Polk and Taylor had now forced an action, and it was enough to get a declaration of war. Polk had his war, and eventually, with the Treaty of Guadalupe-Hidalgo, would add 525,000 square miles to the nation. It was a war of pure territorial acquisition, a war of Manifest Destiny.

The East Texas Historical Association provides this column as a public service. Scott Sosebee is a Professor of History at Stephen F. Austin State University and the Executive Director of the Association. He can be contacted at sosebeem@sfasu.edu or via www.easttexashistorical.org.