When in 1846 President James K. Polk claimed that American soldiers had been attacked on US soil by Mexican troops, he used this as a premise to go to war with American's neighbors to the south. In truth, the title to the land where the skirmish had occurred was disputed, but since Polk had been attempting to acquire more land from the Mexicans, the incident was a convenient reason to attempt to take the land by force rather than by negotiation.

During the first few weeks following the declaration of war a frenzy of support for the war swept the country. Jingoistic sentiments abounded in some quarters, especially in the south where many saw this as an opportunity to acquire more land for potential slave states. Two hundred thousand men responded to Polk's call for 50,000 volunteers. Even in northern states like New York, placards bore the slogan "Mexico or Death." Some newspaper editorials claimed that the war would benefit the Mexican people. For example, a Boston Times article said that an American victory "must necessarily be a great blessing," because it would bring "peace into a land where the sword has always been the sole arbiter between factions" and would introduce "the reign of law where license has existed for a generation."

But not everyone saw things that way. At the start of the war, a small but notable group of intellectuals, clergymen, pacifists, abolitionists, and politicians from bot the Whigs and Democrats saw the war as a form of bullying, a brutal aggression against what one politician called a "poor, feeble, distracted country." Congregationalist minister Theodore Parker declared that if the "war be right then Christianity is wrong, a falsehood, a lie." Abolitionist William Lloyd Garrison's militant newspaper, the Liberator, expressed open support for the Mexican people. Garrison wrote, "Every lover of Freedom and humanity throughout the world must wish them the most triumphant success."

Most Whigs supported the war--in part, because two of the leading American generals, Zachary Taylor and Winfield Scott, were believed to be members of their party. The Whigs also recalled how opposition to the War of 1812 had destroyed the Federalist party and they didn't want the same thing to happen to them. But many prominent Whigs, from the South as well as the North, openly expressed opposition. For example Ohio Senator Thomas Corwin denounced the war, stating: "If I were a Mexican I would tell you, 'Have you not room enough in your own country to bury your dead.'" Even Henry Clay declared, "This is no war of defense, but one of unnecessary and offensive aggression." One of Clay's sons would enlist and would be killed in the war.

A freshman Congressman from Illinois named Abraham Lincoln also lashed out against the war. He called it immoral, proslavery, and a threat to the nation's republican values. In Congress, he proposed what became known as the "Spot Resolution," a motion demanding that President Polk identify the precise spot on which Mexicans had "shed American blood on American soil." This made Lincoln unpopular, even at home, where one of Lincoln's constituents branded him "the Benedict Arnold of our district." Lincoln was not renominated by his own party in the next congressional election.

As newspapers informed their readers about the hardships of life on the front, public enthusiasm for the war began to wane. The war did not turn out to be the romantic exploit that Americans envisioned. Troops complained about poor conditions including rations that were "green with slime." Disease was rampant, including diarrhea, amoebic dysentery, measles, and yellow fever. Seven times as many Americans died of disease and exposure as from battlefield injuries. Of the 90,000 Americans who served in the war, only 1,721 died in action, while 11,155 died from disease and exposure to the elements.

Public support for the war was further eroded as reports came out describing brutality against Mexican civilians. There were a number of wartime atrocities reported. For example, after one of their members was murdered, the Arkansas volunteer cavalry surrounded a group of Mexican peasants and began what was later described as an "indiscriminate and bloody massacre of the poor creatures." A young lieutenant (and future Civil War Union General) named George G. Meade reported that volunteers in Matamoros robbed the citizens, stole their cattle, and killed innocent civilians "for no other object than their own amusement." Even commanding General Winfield Scott had said that if only a tenth of these stories were true, it was enough "to make Heaven weep, & every American of Christian morals blush for his country."

Dissent even made its way among the ranks of the US soldiers. A group of enlisted Irish-Catholic Americans, were angered by the desecration of Catholic churches by their fellow soldiers. They deserted to the Mexican side, formed the San Patricio Battalion, and fought against the American army. At Churubusco, 65 members of the battalion (which also consisted of foreign nationals resident in Mexico) were captured. Fifty were executed and 11 others were punished with fifty lashes apiece and the letter D (for deserter) was branded on their cheeks.

Perhaps one of the most famous acts of protest against the war was made by a young poet named Henry David Thoreau. On July 23, 1846, the constable of Concord, Massachusetts, arrested Thoreau for failure to pay the state poll tax (a head tax on male citizens between the ages of 21 and 70). The constable had even offered to pay the tax if Thoreau was short of money, but Thoreau insisted that he refused to pay on principle, as a protest against his country's involvement in the Mexican War. The constable then placed Thoreau in the local jail. Thoreau spent only a single night in jail because one of his relatives paid his tax, much to his disgust.

In response to his arrest Thoreau wrote an essay entitled "Civil Disobedience." In it he declared that if all citizens who opposed the Mexican War followed his example and went to jail for their beliefs, the government could be forced to end the conflict. He argued that it was the duty of every individual to protest a government policy, even if it had been adopted with majority consent, when it conflicted with moral law. "Any man more right than his neighbor," he argued, "constitutes a majority of one." Thoreau called for a type of disobedience that disrupted the everyday workings of society and highlighted the moral issues at stake, without resorting to violence.

Frederick Douglass opposed the war and lamented the weakness of the anti-war movement. He said, "The determination of our slave-holding president, and the probability of his success in wringing from the people, men, and money to carry it on, is made evident by the puny opposition arrayed against him. None seem willing to take their stand for peace at all risks."

Despite the protests, Polk was able to manipulate Whigs into supporting appropriations for the war once it had already started , but not everyone went along. Ohio Congressman Joshua Giddings led a group of dissenters in Washington D.C. Giddings called the war with Mexico "an aggressive, unholy, and unjust war" and voted against supplying soldiers and weapons. He said: "In the murder of Mexicans upon their own soil, or in robbing them of their country, I can take no part either now or hereafter. The guilt of these crimes must rest on others. I will not participate in them."

In the Senate on February 11, 1847, Whig leader Robert Toombs of Georgia declared: "This war is nondescript ... We charge the President with usurping the war-making power ... with seizing a country ... which had been for centuries, and was then in the possession of the Mexicans. ... Let us put a check upon this lust of dominion. We had territory enough, Heaven knew."

Democratic Representative David Wilmot introduced the Wilmot Proviso, which would prohibit slavery in new territory acquired from Mexico. Wilmot's proposal passed the House but not the Senate.

Northern abolitionists attacked the war as an attempt by slave owners to strengthen the grip of slavery and thus ensure their continued influence in the federal government.

The U.S. Army, under Major General Winfield Scott, invaded the Mexican heartland beginning with a landing at the port of Veracruz on March 9, 1847. Scott later captured the capital of Mexico City in September 1847. Polk sent his peace envoy, Nicholas Trist to negotiate a treaty, but later fired him. Despite this, Trist ignored the order and successfully concluded the 1848 Treaty of Guadalupe Hidalgo which ended the war. Mexico ceded present-day Texas, California, Nevada, and Utah as well as parts of present-day Arizona, Colorado, New Mexico, and Wyoming. The U.S. agreed to pay $15 million (equivalent to $486 million in 2026) for the physical damage of the war and assumed $3.25 million of debt already owed by the Mexican government to U.S. citizens. Mexico relinquished its claims on Texas and accepted the Rio Grande as its northern border with the United States.

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