Mexican-American War Begins as Deliberate Land Grab for Slavery Expansion

President James K. Polk obtains a declaration of war against Mexico after deliberately provoking hostilities by sending American troops into disputed territory between the Nueces River (Mexico’s claimed boundary) and the Rio Grande (Texas’s claimed boundary) in January 1846. When Mexican forces fire on American troops on April 25, 1846, Polk tells Congress that Mexico has “shed American blood upon American soil,” securing his desired war declaration on May 13, 1846, despite knowing the incident occurred in contested territory. The war represents a transparently imperialist land grab designed to acquire Mexican territory—particularly California and the Southwest—to expand slavery and enrich land speculators, demonstrating how executive manipulation of international incidents can manufacture justification for wars of territorial conquest serving elite economic interests.

The war faces immediate opposition from many Whigs and abolitionists who recognize Polk’s deception. The Whig-controlled House votes 85 to 81 to censure Polk for having “unnecessarily and unconstitutionally” initiated the war. Illinois Representative Abraham Lincoln introduces the “Spot Resolutions,” demanding to know the precise spot on U.S. soil where American blood was shed, challenging Polk’s casus belli. Ulysses S. Grant, a young officer who participates in the war, later labels it “one of the most unjust ever waged by a stronger nation against a weaker nation.” The antislavery faction argues correctly that Polk deliberately provoked hostilities to acquire more slave territory—Polk had already unsuccessfully offered Mexico $25 million to purchase the disputed territory plus California and everything in between, and Mexico’s refusal led him to pursue military conquest. Democrats, especially from the Southwest, strongly support the war, revealing the sectional and economic motivations underlying the conflict.

The February 2, 1848 Treaty of Guadalupe Hidalgo forces Mexico to cede nearly all territory now included in New Mexico, Utah, Nevada, Arizona, California, Texas, and western Colorado for $15 million and U.S. assumption of citizen claims—approximately 525,000 square miles, representing over half of Mexico’s territory. Mexico had denounced even the 1845 annexation of Texas as “an act of aggression, the most unjust which can be found recorded in the annals of modern history.” The territorial acquisition immediately triggers fierce debates over slavery’s expansion: on August 8, 1846, Congressman David Wilmot introduces the “Wilmot Proviso” rider stipulating that “neither slavery nor involuntary servitude shall ever exist” in any acquired territory. While Southern senators block the Proviso, the debate it provokes festers until the Confederacy’s 1865 defeat. The Mexican-American War exemplifies kakistocracy through manufactured justification for aggressive war, executive deception of Congress and the public, and the subordination of international law and human rights to the economic interests of slaveholders and land speculators seeking territorial expansion.