President James K. Polk

On May 13, 1846, the United States of America declared war on Mexico. In a congressional vote that carried strong Southern support, it was only the second time in the history of the republic that Congress voted for war.

The war sprang from two main reasons: the desire of the administration of James K. Polk to acquire California and other Mexican regions, and the unsettled territorial arrangements following the United States' annexation of the Republic of Texas in 1845. Mexico claimed that the border between the United States and Mexico was the Nueces River, while Texans and the Polk administration claimed the border ran along the Rio Grande River, further south.

In order to settle these disputes, Polk sent John Slidell as a representative to Mexico. The negotiations broke down, and soon both sides prepared for war. Polk ordered Gen. Zachary Taylor into the disputed zone, and once the Mexican army attacked to repel the "invasion," Polk had his casus belli.

On Monday, May 11, Polk addressed Congress and asked for a declaration of war: “After reiterated menaces, Mexico has passed the boundary of the United States, has invaded our territory and shed American blood upon American soil. She has proclaimed that hostilities have commenced, and that the two nations are now at war.” The fact that the “American soil” in question had been a disputed zone was left intentionally vague and ignited passions against the Mexican government.

That evening, the House of Representatives voted 174 to 14 in favor of war, and the following day the Senate voted 40 to 2.

Polk biographer Walter R. Borneman writes: “The following afternoon, May 13, 1846, a congressional delegation called on the president and presented him with the act declaring war against Mexico. Polk read it in the presence of the congressmen and then signed it. Afterward, he met with Secretary of War (William) Marcy and General Winfield Scott to discuss troop dispositions and the many volunteers to be raised.”

The nation responded positively to the declaration, with broad outpourings of support in various American cities. Historian Robert W. Johannsen writes: “Congress's action unleashed enthusiastic public demonstrations throughout the country. Twenty thousand Philadelphians gathered to voice their support for the war, and in New York even more thronged a mass meeting to cheer. ... In Richmond an immense crowd gathered at the city hall and martial music was heard in the streets ....”

Despite the general popular support for the war, the conflict soon exacerbated the sectional differences of the United States, with Northerners expressing fear that new territories acquired from the war would eventually be admitted to the Union as slave states. By contrast, Southerners applauded the idea. By August, this difference became a major point of contention in Congress.

Democratic Rep. David Wilmot of Pennsylvania proposed that a rider should be attached to a war appropriations bill that stated, “as an express and fundamental condition of the acquisition of any territory from the Republic of Mexico ... neither slavery nor involuntary servitude shall ever exist in any part of said territory.”

The Wilmot Proviso, as it was dubbed, repeatedly passed in the House but repeatedly failed in the Senate. During his one term in congress, Whig Abraham Lincoln adamantly supported the proviso. After the United States conquered Mexico City, the war ended with the Treaty of Guadalupe Hidalgo in February 1848. As a result of the treaty, the United States received all or part of what would become seven states.

The acquisition of this new territory opened the floodgates for sectional passions in the United States. The great defining national political question over the next decade would deal with whether or not slavery would expand into those territories.

For his part, Ulysses S. Grant, who served as a junior officer in the Mexican War and would later go on to be a general in the Civil War and eventually president of the United States, disliked the war from the beginning on moral grounds. He wrote in his memoirs: “For myself, I was bitterly opposed to the measure, and to this day regard the war as one of the most unjust ever waged by a stronger nation against a weaker nation.”

Grant also believed that the spread of slavery into the territories after the war and the subsequent Civil War were a divine retribution for the injustice of the Mexican War: “Nations, like individuals, are punished for their transgressions. We got our punishment in the most sanguinary and expensive war of modern times.”

Cody K. Carlson holds a master's degree in history from the University of Utah and currently teaches at Salt Lake Community College. EMAIL: [email protected]