The Mexican War was the first American conflict to be fought primarily on foreign soil and the first in which American troops occupied a foreign capital. Inspired by the expansionist fervor of manifest destiny, a majority of Americans supported the war. They were convinced, as Herman Melville put it in his novel White-Jacket (1850), that since Americans “bear the ark of Liberties” for all mankind, “national selfishness is unbounded philanthropy... to the world.” But a significant minority in the North dissented, fearing that far from expanding the “great empire of liberty,” the administration’s real aim was to acquire new land for the expansion of slavery. Ulysses S. Grant, who served with distinction in Mexico, later called the war “one of the most unjust ever waged by a stronger nation against a weaker nation,” an indication that the United States was beginning to behave like “European monarchies,” not a democratic republic. Henry David Thoreau was jailed in Massachusetts in 1846 for refusing to pay taxes as a protest against the war. Defending his action, Thoreau wrote an important essay, “On Civil Disobedience,” which inspired such later advocates of nonviolent resistance to unjust laws as Martin Luther King Jr. “Under a government which imprisons any unjustly,” wrote Thoreau, “the true place of a just man is also a prison.”
Among the war’s critics was Abraham Lincoln, who had been elected to Congress in 1846 from Illinois. Like many Whigs, Lincoln questioned whether the Mexicans had actually inflicted casualties on American soil, as Polk claimed, and in 1847 he introduced a resolution asking the president to specify the precise “spot” where blood had first been shed. But Lincoln was also disturbed by Polk’s claiming the right to initiate an invasion of Mexico. “Allow the president to invade a neighboring country whenever he shall deem it necessary to repel an invasion,” he declared, “and you allow him to make war at pleasure.... If today he should choose to say he thinks it necessary to invade Canada to prevent the British from invading us, how could you stop him?” Lincoln’s stance proved unpopular in Illinois. He had already agreed to serve only one term in Congress, but when Democrats captured his seat in 1848, many blamed the result on Lincoln’s criticism of the war. But the concerns he raised regarding the president’s power to “make war at pleasure” would continue to echo in the twentieth and twenty-first centuries.