The debate over the Mexican War of September 1846 to April 1847 was made extraordinarily passionate because the war was the focal point for rising American concerns about the institution of slavery. The war, when won, would introduce vast new territories to the United States. The debate then became: Should these territories and future states be slave-holding or free?

New Territories and New Concerns

President James Polk, a believer in Manifest Destiny – the idea that Americans were preordained by God to possess the entire continent – went to war essentially because Mexico would not sell the United States California and was intent on disputing ownership of Texas and large portions of the current American Southwest. After America was victorious in the war, Mexico was forced to cede California to the United States, and what are now the states of New Mexico, Nevada, Utah, Arizona and parts of Wyoming and Colorado. However American political parties were divided in the debate about whether to allow slavery in these new territories.

A Sharp Divide

A focal point of the debate was the Wilmot Proviso, introduced by Democratic Rep. David Wilmot of Pennsylvania in 1846. The Wilmot Proviso declared that “neither slavery nor involuntary servitude shall ever exist” in any territory acquired from Mexico. The Wilmot Proviso passed the House, but not the Senate; however, the Proviso did underscore the sharp split in sentiments about slavery in the United States, since Northern Democrats and Northern Whigs were for it, while Southern Democrats and Whigs were sharply opposed.

A Power Grab by Slave-Holders

In some ways, the political debate was not over slavery alone, since if these new territories and states were allowed become to become slave-holding, the South, with its smaller population, would control a disproportionate voting bloc in Congress and be able to exert its power in other areas, as well. This was the fear of American abolitionists, a broad-based anti-slavery group that had grown in force since the 1830s. While the war in Mexico proceeded too rapidly for anti-war sentiment to coalesce into an organized force, abolitionists provided strong opposition, since they considered the conflict a deliberate attempt by Polk (a slave-holder himself) to gain new territory in order to expand slavery.

The Rise of Abolitionism

While abolitionists had previously been considered too radical by many Americans – some abolitionists wanted to secede from the Union – the debate over slavery engendered by the Mexican War gained them new respectability. A supporter of the abolitionists, Henry David Thoreau briefly went to jail for refusing to pay a local Massachusetts tax as protest against the war and slavery. His essay “Civil Disobedience” became a classic, and the abolitionists were now center stage as the country headed toward the Civil War.