The Mexican War, which lasted from 1846 until 1848, resulted in the U.S. acquisition of 525,000 square miles of territory. While the U.S. won the war decisively, the country had plenty of internal disagreement about the war's justification and its outcome. Many groups of Americans, including literary figures and politicians, voiced opposition to the Mexican War because of concerns about slavery and Manifest Destiny.

Whig Politicians

While the Mexican War was supported among some political groups, Northern Whigs detested the conflict. One of the major Whig concerns was about the constitutionality and truthfulness of President Polk's actions. The war began when Polk informed Congress that Mexican troops had fired on American troops in Texas, and that the president needed a Declaration of War to defend the country. Polk, however, failed to inform Congress that the Mexicans fired first because Americans had overstepped into Mexican territory. Neglecting this small but important piece of information incensed Northern Whigs, who labeled the Mexican War "Mr. Polk's War." Opponents included Congressman Abraham Lincoln, a Whig who would become president more than a decade later.

Literary Figures

Leading literary figures in the United States, many of whom were part of the Transcendentalist movement, opposed the Mexican War on moral grounds. The Transcendentalists believed that societal institutions -- like governments -- were morally bankrupt. They saw this through actions like the continuation of slavery and the Mexican War. The transcendentalists felt a moral obligation to protest unjust policies, and so many, like Henry David Thoreau, loudly opposed the war. Thoreau, for example, was imprisoned for refusing to pay his taxes and authored his "Civil Disobedience" during the war.

Abolitionists

Opponents of slavery were highly critical of the Mexican-American War. Abolitionists feared that slave owners would use any territory gained from Mexico to expand slavery. These fears were not unfounded. When Texas was annexed just years earlier, and the state legalized slavery. Notably, Northern abolitionists like John Quincy Adams did not oppose territorial expansion altogether. They merely opposed it when they thought it would expand slavery. Just years before the Mexican War, for example, abolitionists like Adams supported the annexation of Oregon, which Adams believed would not be home to slavery.

Journalists

While newspapers provided informative coverage of the ongoing Mexican War, some editors of major publications expressed reservations about the aims of the conflict. In the process of reporting on the war, many newspaper editors learned about terrible atrocities on the war's front. This included brutalities like massacres and robberies committed against Mexican citizens. This view conditioned many newspaper editors -- though not all -- to oppose the war and write critical editorials.