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 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 . . . The Texans managed to maintain their independence for almost a decade even as they petitioned repeatedly for annexation by the United States . . .

[In 1845 the new US President James Polk] sent an envoy to Mexico City to try to intimidate the unstable government into accepting the Rio Grande border and selling New Mexico and California to the United States. Meeting refusal, Polk ordered General Zachary Taylor to lead a contingent of American soldiers to the Rio Grande. Polk hoped this move would provoke an incident that would enable the United States to declare war and seize the territory that Mexico refused to sell . . . the Mexican commander on the south bank of the Rio Grande created an incident by sending troops across the river to attack an American patrol, killing eleven of them . . .

American armies had a long string of military successes that gave the United States control of New Mexico and California, and they captured Mexico City itself by September 1847. Nevertheless, the growing list of casualties and reports of atrocities by American soldiers against Mexican civilians and of savage attacks by Mexican rancheros (guerrillas) on American soldiers intensified antiwar sentiment in the United States . . .

By the summer of 1847, even journalists employed by pro-war newspapers “found themselves forced to report on and condemn American atrocities that left them questioning their assumptions about American morality.”