“I do not think there was ever a more wicked war than that waged by the United States on Mexico,” said Ulysses S. Grant in 1879, more than thirty years after he had fought in that war as a young lieutenant. As he was dying of cancer in 1885, Grant re- asserted that the American war against Mexico was “one of the most unjust ever waged by a stronger against a weaker nation.” Like the adventure in Iraq more than a century later, it was a war of choice, not of necessity, a war of aggression that expanded the size of the United States by nearly one quarter and reduced that of Mexico by half. And in a glaring example of unintended consequences, the issue of slavery in this new American territory set in motion a series of events that would produce a much bigger war fifteen years later that nearly tore apart the United States.
Two principal forces impelled Americans toward what historian Amy Greenberg as well as General Grant consider a wicked war. The first was the annexation of Texas in 1845. Soon after Mexico won its independence from Spain in 1821, the new government offered American settlers large land grants to move into its sparsely populated northern province of Tejas. The Mexican government soon had reason to regret this policy. The Americans brought slaves in defiance of a Mexican law abolishing the institution. They also defied Mexican efforts to regulate land claims and political activities.
Despite Mexican attempts to ban further immigration, by 1835 30,000 Americans lived in Texas, where they outnumbered native Mexicans (tejanos) by six to one. Determined to establish their own government, the American Texans met at a village appropriately named Washington in 1836 and declared their independence. After suffering the slaughter of all 187 defenders of the mission in San Antonio called the Alamo and another massacre of more than three hundred captives at the city of Goliad, the Texans defeated a larger Mexican army at San Jacinto on April 21, 1836. The Texans captured the Mexican commander Antonio López de Santa Anna and forced him to sign a treaty recognizing the independent republic of Texas.
Although the Mexican Congress repudiated this treaty, the Texans managed to maintain their independence for almost a decade even as they petitioned repeatedly for annexation by the United States. These petitions, however, ran into a snag in Washington, where the growing controversy over the extension of slavery temporarily derailed the drive for annexation. So did the opposition of most Whigs to the idea of continual territorial expansion, which was embraced by the “Young America” faction that increasingly dominated the Democratic Party and generated the second impulse toward the Mexican War. The “Manifest Destiny” of the United States was to possess the whole of North America, proclaimed John L. O’Sullivan of The Democratic …