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The twenty-day siege of the Mexican city of Veracruz in March 1847; painting by William Henry Powell, 1867

“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 Review in 1845. “Yes, more, more, more!…till our national destiny is fulfilled and…the whole boundless continent is ours.”1

Set against this demand for expansion of territory was the Whig philosophy of “internal improvements” by building up the infrastructure of transportation, education, and economic development within the existing borders of the United States. “Opposed to the instinct of boundless acquisition stands that of Internal Improvement,” wrote the Whig journalist Horace Greeley. “A nation cannot simultaneously devote its energies to the absorption of others’ territories and improvement of its own.”2

The foremost exponent of this Whig position was Henry Clay, a three-time loser as a presidential candidate who nevertheless was an immensely influential political figure in the first half of the nineteenth century. His third try for the nation’s highest office came in 1844, when he was defeated by the crosscurrents of Manifest Destiny and the antislavery opposition to the annexation of Texas. Clay’s presumptive Democratic opponent in this election was Martin Van Buren, also making his third bid for the presidency after winning in 1836 and losing four years later. Both Clay and Van Buren came out against annexation of Texas in letters published simultaneously on April 27, 1844. As matters turned out, however, these letters sealed their fate. The pro-annexation current ran so strongly in the Democratic Party that it nominated the dark-horse candidate James K. Polk of Tennessee on a platform that endorsed the acquisition not only of Texas but also of the Oregon Territory through Canada up to the border of Russian Alaska above the 54th parallel.

Despite the slogan “Fifty-four forty or fight!” that seemingly courted war with Britain over possession of British Columbia, it was the Texas issue that caught fire with the electorate. Annexation sentiment was especially strong in the South, which welcomed the prospect of a huge new slave state. To stem the stampede of many Southern Whigs to Polk on this issue, Clay published two more letters in July explaining that while he still opposed annexation if it would mean war with Mexico, he would acquiesce if it could be accomplished without war and with consensus support of Americans. This waffling probably cost him the election. Enough antislavery Whig voters in New York abandoned Clay and cast their ballots for the tiny Liberty Party to give that state—and therefore the presidency—to Polk by a margin of five thousand votes.


Clay and Polk are two of the five principal leaders around whom Greenberg organizes her book. A sixth figure enters the story briefly: Polk’s wife Sarah, a politically savvy woman who became his alter ego. “Were it not for her political skills, James Polk might never have won office,” writes Greenberg with perhaps a touch of hyperbole. “Members of both parties knew that Polk’s election would be a two-for-one…. Together they made, if not a good president, certainly a successful one.”

Polk’s success consisted mainly in presiding over the acquisition of more territory than any other president. He moved quickly to complete the annexation of Texas, which came in as the twenty-eighth state (and fifteenth slave state) in 1845. Polk then compromised with Britain to establish the northern border of the Oregon Territory at the 49th parallel. Having pledged to fight for a border of fifty-four forty, Polk angered many Northern Democrats with his refusal to risk war with Britain while being willing to provoke war with Mexico by annexing Texas and insisting on a border at the Rio Grande River instead of the old Mexican border at the Nueces River, which effectively doubled the size of Texas now claimed by the United States.

The new president 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 (which included Lieutenant Ulysses S. Grant) 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. If not, Polk intended to ask Congress for a declaration of war anyway. In the event, 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.

Even before this news reached Washington on May 9, Polk’s cabinet had decided to request a declaration of war. Now the president had his casus belli. He sent a message to Congress asking not for a declaration of war as such, but for a resolution asserting that war already existed because Mexico “has invaded our territory and shed American blood upon the American soil.” As Greenberg notes, “None of it was true—but Polk didn’t consider it lies.” He believed that “a greater truth” was at stake: “As war exists, and notwithstanding all our efforts to avoid it, exists by the act of Mexico herself, we are called upon by every consideration of duty and patriotism to vindicate with decision the honor, the rights, and the interest of our country.”

Most Democrats were enthusiastic proponents of this war; most Whigs were opposed, branding it “Mr. Polk’s War.” (Greenberg prefers to call it “Mr. and Mrs. Polk’s War,” the title of Part Two of her book.) Congressional Democrats attached the declaration that war existed by the act of Mexico as a preamble to a bill to authorize funds and supplies for American soldiers who were now in harm’s way. This was a cynical ploy to force Whigs to vote yes or be forever tainted by a refusal to support the troops. It worked. Only two Whigs in the Senate and fourteen in the House (including former president John Quincy Adams) voted against the declaration.

For the same reason that most Whig congressmen felt compelled to vote yes, many other Whigs volunteered to fight in a war they deplored in order to prove their patriotism. Two of the most prominent were Henry Clay Jr. and John J. Hardin (a colleague of Abraham Lincoln in Illinois Whig politics who preceded Lincoln as a congressman from the Springfield district). Clay and Hardin became colonels respectively of Kentucky and Illinois regiments. Hardin is the third of Greenberg’s main characters, a charismatic politician who believed in Manifest Destiny despite his Whig allegiance. When war came in 1846, he was the first man from Illinois to enlist. Henry Clay Jr. was considerably less enthusiastic but no less determined. His departure from home and family was poignant and painful. “How bitter it was,” writes Greenberg, “that Henry Junior was risking death for a president his father detested and a conflict he despised.”


Death came to both Clay and Hardin at the Battle of Buena Vista in northern Mexico in February 1847. This battle was the most remarkable of American victories in the war, fought against odds of more than three to one. 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. Total American deaths of 13,283 (seven eighths of them from disease) represented 17 percent of all American soldiers, the highest rate for any war except the Civil War. Poorly disciplined volunteer soldiers occupying Mexican cities “committed atrocities against Mexican civilians that would come to shock Americans back home,” Greenberg writes. Lieutenant Grant wrote to his fiancée Julia Dent from Monterrey:

Some of the volunteers and about all of the Texans seem to think it perfectly right…to murder them where the act can be covered by the dark…. I would not pretend to guess the number of murders that have been committed upon the persons of poor Mexicans and the soldiers, since we have been here, but the number would startle you.

An 1848 political cartoon attacking a Whig presidential candidate—probably Winfield Scott—for his service as a general during the Mexican War

By the summer of 1847, according to Greenberg, 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.”

Although most voters in Western and Southern states supported the war, as the months went by and no end appeared in sight, antiwar sentiment increased even though American arms experienced nothing but victory. The slavery issue compounded the controversy. Much antiwar opinion was fueled by the suspicion that the principal purpose of the conflict was to acquire more territory for slavery. As early as August 1846 a Pennsylvania congressman named David Wilmot introduced an amendment to an appropriations bill for the war stating that “as an express and fundamental condition to the acquisition of any territory from the Republic of Mexico…neither slavery nor involuntary servitude shall ever exist in any part of said territory.”3

This famous “Wilmot Proviso” framed the national debate over slavery for the next fifteen years. Nearly all Northern Democrats joined all Northern Whigs in the majority that passed the amendment, while Southern Democrats and Southern Whigs voted against it. In the Senate, greater Southern strength defeated the proviso. This outcome, which was repeated several times in the next two Congresses, marked an ominous wrenching of the party division, between Whigs and Democrats, into a sectional division between free and slave states that foreshadowed the political breakdown that led to secession and war in 1861.

As the war dragged on despite American conquest of a huge swath of Mexico, Polk wanted to negotiate peace and complete the accomplishment of his territorial goals before he lost control of events. Enter Nicholas Trist, the fourth of Greenberg’s characters and the real hero of her narrative. Probably not one in a hundred Americans today could identify Trist, yet he was prominent in his time. A protégé of the elderly Thomas Jefferson, who supervised his legal education and made him his private secretary, Trist also married Jefferson’s granddaughter. Later he served as Andrew Jackson’s personal secretary for a time, and in 1845 Jackson persuaded Polk to appoint Trist as chief clerk of the State Department—the equivalent of assistant secretary of state.

Fluent in Spanish, a Democrat and expansionist, Trist seemed the ideal person to negotiate a peace with Mexico that would force its government to yield half its country to the United States. Polk appointed him as a special envoy to accompany General Winfield Scott’s army, which was closing in on Mexico City. Trist was authorized to offer Mexico up to $20 million in return for the Rio Grande boundary of Texas plus New Mexico and California (embracing the present-day states of California, Nevada, and Utah, most of Arizona and New Mexico, and part of Colorado). If Mexico would also throw in Baja California, Trist could pay up to $30 million.

During Trist’s sojourn in Mexico, a number of things happened that set up a dramatic confrontation between the envoy and the president who sent him there. Scott’s army captured Mexico City in September 1847 and drove President Santa Anna and his army away to Guadalupe Hidalgo, where Santa Anna refused to capitulate or negotiate despite the hopelessness of his cause. American military success whetted the appetite of some Manifest Destiny expansionists for more of Mexico than Polk had initially contemplated, perhaps even “all Mexico.”

Polk also began to think that he should demand more territory. But Trist found himself questioning the morality and justice of American policy. His dispatches indicated a growing softness toward Mexico and an unwillingness to go beyond the original territorial goals he had been instructed to achieve. In October 1847 Polk decided to recall Trist and send a new envoy to extract harsher terms. Trist ignored the recall order, at the risk of his career, and in February 1848 he finally negotiated the Treaty of Guadalupe Hidalgo with Santa Anna’s successor, which carried out Polk’s initial territorial goals (minus Baja California).

The arrival of this treaty in Washington presented Polk with a dilemma. He was furious with Trist and tempted to repudiate his treaty in order to force greater concessions from Mexico. But since the autumn of 1847 a growing antiwar movement had begun to threaten to crush Polk between the millstones of Democrats’ “all Mexico” clamor and Whig pressures for “no Mexico.” In November 1847 Henry Clay, still mourning the death of his son, broke his silence on the Mexican War with a powerful antiwar speech in Lexington, Kentucky, that received national publicity.

Among others, it solemnly affected Abraham Lincoln, Greenberg’s fifth main character, who happened to be in Lexington visiting his in-laws while on his way to Washington to take the congressional seat to which he had been elected in 1846. Lincoln heard Clay denounce an “unnecessary” war of “offensive aggression” that had produced “sacrifice of human life…waste of human treasure…mangled bodies…death, and…desolation” in a conflict “actuated by a spirit of rapacity, and an inordinate desire for territorial aggrandizement.” Clay also endorsed the Wilmot Proviso and insisted that the United States must not “acquire any foreign territory whatever, for the purpose of introducing slavery into it.”

Clay’s speech inspired many antiwar meetings around the country and emboldened Whigs to speak out more vigorously against Mr. Polk’s War in the congressional session that began in December 1847. One of those Whigs was Abraham Lincoln. Greenberg maintains that before hearing Clay’s speech, Lincoln had said little about the Mexican War or about slavery, and that Clay galvanized him to take up the antiwar and antislavery cause for the first time. Whether she is right is debatable, but it is quite true that the freshman congressman achieved national exposure with his “spot resolutions” and speeches in the House advocating these resolutions in December 1847 and January 1848. The resolutions demanded from Polk a description of the exact “spot” where Mexican soldiers shed American blood to start the war, suggesting instead that American soldiers shed Mexican blood on Mexican soil.

The political turmoil surrounding the debates about responsibility for the war forced Polk to conclude that he should get the controversy behind him by submitting Trist’s treaty to the Senate for ratification rather than try to negotiate a new treaty. In the Senate, several Democrats who wanted more Mexican territory and several Whigs who wanted none voted against ratification, but enough senators of both parties voted in favor to pass the treaty with four votes to spare. “Had Nicholas Trist not concluded that justice required him to disobey his president and negotiate a lenient peace treaty with Mexico,” writes Greenberg, “no Treaty of Guadalupe Hidalgo would have emerged in 1848.” Polk fired Trist from his job in the State Department and even withheld his pay for the time he spent in Mexico. Clay also forfeited any chance for another presidential run in 1848 by jettisoning his political base in the South and West, where the war and conquest remained popular. And the Whigs lost Lincoln’s congressional district in 1848, in part because of the unpopularity there of Lincoln’s antiwar speeches.

Greenberg’s narrative is the best account we have of the politics of Mr. Polk’s War. Yet the reader should be wary of her penchant for sweeping declarations that sometimes seem to go beyond the evidence. Was Sarah Polk really “one of the most powerful First Ladies in history”? Is it actually true that if it had not been for Henry Clay’s Lexington speech against the war, “Lincoln might not have opposed the war at all”? Perhaps one can accept that Nicholas Trist was “the only man to single-handedly bring an American war to a close,” but can we agree that John Hardin’s death at Buena Vista “removed a key obstacle from Lincoln’s rise to power”? And perhaps the assertion that the Mexican War “decisively broke with the past, shaped the future, and to this day affects how the United States acts in the world” claims too much.

These statements concern matters of interpretation and emphasis that, by their nature, cannot be proven right or wrong, although for many readers this American political conflict over the legitimacy of a bloody war recalls many of the debates over American military intervention since the Vietnam War of the 1960s. But Greenberg’s assertion that “the US-Mexican War had the highest casualty rate of any American war” is quite definitely wrong. The long-accepted figure of 620,000 Civil War soldier deaths, 21 percent of the three million soldiers and sailors who fought in that war, exceeds the 17 percent of American soldiers who died in the Mexican War. The number of Mexican deaths is unknown. More new research by the demographic historian J. David Hacker finds that the total number of Civil War dead was probably about 750,000, or approximately 25 percent.4

It is natural for a historian to claim preeminent significance for her or his subject, and Greenberg has certainly made a strong case for the importance of hers. She also places it in a setting that helps explain why another war that almost destroyed the United States came in 1861, in considerable part the consequence of what happened from 1846 to 1848. If one can read only a single book about the Mexican-American War, this is the one to read.