This week in 1846 saw the outbreak of one of the most obscure, consequential, and unjust wars in U.S. history: The Mexican American War, which in two years resulted in the U.S. becoming a continental power, at the expense of its weaker southern neighbor—something even American heroes like Abraham Lincoln and Ulysses S. Grant regarded as a grave injustice.
The war began under the equally obscure but history-making presidency of James K. Polk, a one-term president with the rare distinction of having fulfilled all his campaign promises—one of which was expanding U.S. territory to the Pacific.
The problem was that Mexican (and to a lesser extent British) territory was in the way. Beginning with the Louisiana Purchase of 1803, which more than doubled the size of the fledging republic, there were several overtures to purchase what was then Spanish territory; in 1825, Andrew Jackson made a sustained effort to buy the northern lands of what was now newly independent Mexico, to no avail.
Meanwhile, Mexico was well aware of its precarious position: Not only was it wracked by political instability and social strife, but it lacked full authority over the rugged, sparsely inhabited lands of the now-American Southwest—especially against the various fiercely independent native tribes that were effectively sovereign. So, in the 1820s, the Mexican government invited Americans to settle and “civilize’ the vast, largely empty plains of present-day Texas; among them were men like Stephen F. Austin, the “Father of Texas“, who brought hundreds of “Anglo” families with him.
The rapid influx of Americans led to them outnumbering Mexicans in their own distant territory, which was already thousands of miles from Mexico’s political base in Mexico City. Aside from cultural and linguistic barriers, a major sticking point—surprise—was slavery: Mexico’s constitution had outlawed the practice decades before the U.S., but the vast majority of American settlers were slaveowners.
In a macabre foreshadowing of what was to come, disputes over slavery—along with the Mexican government’s effort to impose property taxes on the fiercely independent American immigrants—led Mexico to close the border with the U.S.—only for American slave owners to continue illegally crossing into Mexico (no need to harp on the irony here).
Escalating matters further, Mexico’s strongman president, Antonio Lopez de Santa Anna, sought to roll back the country’s federal system in favor of centralized power; this upset the quasi-independent “Texans”, and when Santa Anna led an army to reign them in, the Texas Revolution broke out, and the Texans, with U.S. support, achieved de facto independence in 1836.
Mexico never recognized this claim—though the U.S. and other foreign powers did—and the border of this new “Republic of Texas” were subsequently unclear and disputed. So, when America made the controversial move of annexing Texas as a state in 1845—hotly debated in Congress and by the public—this brought the dispute to what was now our border.
After yet another failed attempt to buy Mexican territory and finding significant opposition to starting a war with its only independent neighbor, Polk essentially egged on Mexico to start hostilities first—by sending a military expedition deep into Mexican territory. Even Grant, who served in the war despite his opposition to it, claims in his Personal Memoirs (1885) that the main goal was to provoke the outbreak of war without attacking first, thereby hindering domestic opposition to the war.
“The presence of United States troops on the edge of the disputed territory farthest from the Mexican settlements, was not sufficient to provoke hostilities. We were sent to provoke a fight, but it was essential that Mexico should commence it. It was very doubtful whether Congress would declare war; but if Mexico should attack our troops, the Executive could announce, “Whereas, war exists by the acts of, etc.,” and prosecute the contest with vigor. Once initiated there were, but few public men who would have the courage to oppose it. … Mexico showing no willingness to come to the Nueces to drive the invaders from her soil, it became necessary for the “invaders” to approach to within a convenient distance to be struck. Accordingly, preparations were begun for moving the army to the Rio Grande, to a point near Matamoras. It was desirable to occupy a position near the largest centre of population possible to reach, without absolutely invading territory to which we set up no claim whatever.”
After Mexican forces engaged what it saw as American invaders, killing or capturing dozens, Polk made his case for war. Many pro-slavery Democrats supported a declaration of war, while many northern “Whigs” remained staunchly opposed—including a freshman Congressman from Illinois named Abraham Lincoln, who challenged Polk’s assertion that American blood had been shed on American soil as “a bold falsification of history.” Within hours, Congress voted to formally declare war against Mexico—one of the few times in history that the U.S. as officially been at war with another country.
Notwithstanding some success on the battlefield, Mexico simply lacked the resources, military experience, and political unity to defend itself against superior American forces. Once its capital was occupied—along with most other major cities—it was clear that the U.S. was victorious and could dictate terms—which unsurprisingly included annexing the northern territories the U.S. had long sought.
(There was actually an “All of Mexico Movement” that sought to take the entirety of Mexico, but it fell apart due in large part to concerns about incorporating millions of inferior Indian and mixed races that comprised the majority of the country’s population.)
In the peace treaty that followed, Mexico ceded to the United States the present-day states of California, Nevada, and Utah, most of New Mexico, Arizona and Colorado, and parts of Texas, Oklahoma, Kansas, and Wyoming.
In return, Mexico received $15 million—$470 million today—which was less than half the amount the U.S. offered before the war; the U.S. further agreed to assume $3.25 million in debts that the Mexican government owed to U.S. citizens ($102 million today).
Aside from its obvious enrichment of the U.S., the war had a huge impact on American domestic politics: A bloody expansion led to a bitter and polarizing debate about whether America was fulfilling its “Manifest Destiny” as an enlightened republic or was instead no different than the imperialist Europeans it claimed to have broken from. Once again, Grant captured the mood in his memoirs:
“For myself, I was bitterly opposed to the measure, and to this day regard the war, which resulted, as one of the most unjust ever waged by a stronger against a weaker nation. It was an instance of a republic following the bad example of European monarchies, in not considering justice in their desire to acquire additional territory.”
The already-violent debate over slavery came to a head as both sides debated which of these vast territories should be “free” or “slave”; it was a cruel irony considering that the war had begun partly because illegal American immigrants insisted on having slaves in an “uncivilized” nation that had long since banned the despicable practice.
In some sense, America’s actions came to haunt it barely a generation later when these disputes over the fate of former Mexican territory furthered the boiling point to the American Civil War—which was led and fought by many veterans of the Mexican American War with tactics and strategies learned from that conflict.
Grant also expressed the view that the war against Mexico had brought punishment on the United States in the form of the American Civil War. “The Southern rebellion was largely the outgrowth of the Mexican war. Nations, like individuals, are punished for their transgressions. We got our punishment in the most sanguinary and expensive war of modern times”.