Non-fiction: Message to Congress Requesting a Declaration of War against Mexico (May 11, 1846)

Context
The message comes after the United States annexed Texas in 1845, a move Mexico refused to recognize because it still claimed Texas as a rebellious province. The core dispute centered on the boundary: the United States recognized the Rio Grande as Texas’s southern border, while Mexico claimed the Nueces River line, leaving a contested strip between. Diplomatic relations were strained by years of unpaid Mexican awards to U.S. citizens and mutual suspicions, and they collapsed when the Mexican government refused formal engagement with a U.S. envoy sent to settle differences.

Breakdown of Diplomacy
Polk recounts sending John Slidell to Mexico with full powers to negotiate both the boundary and outstanding claims. Despite this conciliatory offer, Polk says, the Mexican authorities refused to receive Slidell or negotiate. He casts this as a breach of international comity and evidence that Mexico was not serious about peace. Meanwhile, he reports, Mexican military preparations accelerated on the frontier, and threats were made against Texas.

Military Developments on the Frontier
As a protective measure, Polk ordered General Zachary Taylor to move forces to the Rio Grande, which he treats as the lawful boundary of Texas and thus of the United States. Mexican troops then concentrated on the opposite bank and, according to Polk, crossed the river to attack an American patrol, killing and capturing U.S. soldiers, and later engaged U.S. positions near Fort Texas. Polk summarizes dispatches showing that blood had been shed and that hostilities had begun without provocation from the United States. He crystallizes this point in the line that Mexico “has invaded our territory and shed American blood upon American soil.”

Legal and Moral Framing
Polk’s argument rests on two pillars: the right of self-defense and the integrity of U.S. sovereignty. By treating the Rio Grande as the legitimate boundary inherited with Texas, he portrays Mexico’s crossing as an invasion. He also invokes the law of nations and the duty of the executive to protect citizens and territory. Longstanding financial claims by U.S. citizens against Mexico, and Mexico’s failure to honor them promptly and fully, are presented as aggravating circumstances that underscore a pattern of injury and bad faith. The refusal to receive Slidell becomes central proof that peaceful resolution had been tried and spurned.

Request to Congress
On this record, Polk declares that “war exists by the act of Mexico herself,” and asks Congress to recognize the state of war and to supply the means necessary to prosecute it. He seeks authority and resources to expand the Army and Navy, call volunteers, and appropriate funds, aiming for a vigorous but decisive campaign. The stated objective is a speedy and honorable peace that ensures security and indemnity, rather than conquest for its own sake.

Purpose and Tone
The message blends urgency with legalistic justification, presenting the administration as restrained and patient in diplomacy yet resolute once attacked. Its rhetoric emphasizes honor, defense, and duty, attempting to unify Congress and the public behind a military response while assigning blame squarely to Mexico.

Significance
Congress quickly acted on Polk’s request, recognizing the war on May 13, 1846. The message thus served as the formal hinge from diplomatic crisis to declared war, shaping public understanding by framing the conflict as a defensive necessity rooted in violated sovereignty and failed negotiation.