Whose Aggression Sparked the Mexican-American War?

On May 13, 1846, Congress declared war on Mexico, beginning the two-year Mexican-American War. The spark for this conflict began 18 days earlier, with a skirmish on April 25 called “the Thornton Affair.” From the Mexican perspective, the war was purely an act of American aggression, flimsily based on a pretext that Mexico began the hostilities. The United States claimed that Mexico was the aggressor. Which nation is responsible for provoking the war’s bloodshed is disputed to this day.

Mexico regarded Texas as a renegade province even after the Texas Revolution established the Republic of Texas, and warned the United States not to annex the territory. Under the guise of “Manifest Destiny”—the belief that America had a divine right to expand its territory—the United States defied Mexico and admitted Texas into the Union on Dec. 29, 1845. Mexico retaliated by breaking off diplomatic relations.

Then, the United States claimed that the southern border of Texas was the Rio Grande River, ignoring Mexico’s long-established Texas border at the Nueces River 150 miles north. President Polk, a fervent supporter of Manifest Destiny, ordered American troops under General Zachary Taylor to march down to the Rio Grande, ostensibly to “protect” the Texas border, but in reality to cause a provocation—which Mexico provided. The Americans knew full well that Mexico would consider Taylor’s soldiers to be invading Mexican territory. On April 25, 1846, a detachment of 67 American soldiers, led by Captain Seth Thornton, was out on patrol to see if Mexican troops had crossed the Rio Grande. They had.

Thornton’s men bumped into a force of 2,000 Mexican soldiers. The Americans fought bravely, but were overwhelmed. The Mexican casualties are not known, but 16 of Thornton’s men were killed and 50—included a wounded Captain Thornton—were captured. One man escaped the battle and reported the news of the “Mexican aggression.”

This was all the excuse President Polk needed. On May 11, his message to Congress complained that a belligerent Mexico had “shed American blood upon American soil,” and asked for a declaration of war. Two days later, Congress obliged, and the Mexican-American War was officially underway.

From the American perspective, the war was a justifiable response to Mexican aggression, and most of the public, press and government supported it wholeheartedly. Not everyone supported the war, however; right from the outset, the war had its American opponents.

The following editorial, written just one month before the war’s end, shows sustained opposition to the war. Note the prescience of this article in claiming that a warlike America, enthralled with its victories in Mexico, was likely to elect a military hero as president: the next two men elected to the presidency, Zachary Taylor and Franklin Pierce, both served as generals in the Mexican-American War. (The intervening president between these two men, Millard Fillmore, was Taylor’s vice-president and only became president when Taylor died in office.)

This editorial was published by the Milwaukee Sentinel (Milwaukee, Wisconsin) on Jan. 20, 1848:

There are many reasons why the war with Mexico is to be deplored, beside the loss of valuable lives which it has occasioned, and the immense amount of money expended in its prosecution. One result of this stupendous offspring of folly and wickedness is in rapid process of development, and every day, in all quarters of the Union, political movements, schemes of conquest and annexation, which but a few years ago would have alarmed every man of common sense in the nation, are discussed and advocated with as much earnestness and zeal, as though the subjugation of the world were the true and only mission confided to “the Model Republic.” As a natural consequence of this wild, reckless spirit, military heroes begin to be considered the only available candidates for the Presidency—civilians are constrained to hide their “diminished heads.” Services in the National Legislature through many years of toil and trial, and the claims of distinguished Statesmanship are of no account in comparison with the glory won on the other side of the Rio Grande by Major Generals, Brigadier Generals, Colonels, &c. Men who up to the year 1846 never dreamt of aspiring to the honor of the chief Magistracy, now think it quite within the compass of their reach, and find enough of advocates to encourage and strengthen them in the opinion.

And what is to be the end of this? Is there not some danger that our Government may become that most odious of gov’ts, a military Oligarchy in effect? One thing is certain, that hereafter officers of the army, and their relations, friends and flatterers, will wield a most important influence in the direction of national affairs, and it is not less certain that such influence will always be exerted in opposition to the policy pointed out and commended to his countrymen by the great and good Washington—the policy that seeks to cultivate peace with all nations, and is satisfied with our present territorial limits. Let this Mexican war be continued and the evil alluded to will increase, it is to be feared, beyond all remedy, and we shall begin to discover that “our manifest destiny” is to tread the same path trodden by the kingdoms and republics that have gone before us—the downward road to national decay, ruin and death.

For more information, visit the Guide to the Mexican War website provided by the Library of Congress.

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