On this day in 1846, Congress declares war on Mexico at the request of newly-minted President James K. Polk. The conflict centered on the independent Republic of Texas, which had opted to join the United States after establishing its independence from Mexico a decade earlier.
The new president wanted Texas as part of the United States, and his predecessor, John Tyler, previously cool to the idea, had a late change of heart and commenced the admission process before he left office. Polk and others saw the acquisition of Texas, California, Oregon, and other territories as part of the nation’s Manifest Destiny to spread democracy over the continent. By the end of Polk’s first calendar year in office, Texas was admitted to the union as a slave state.
Mexico considered the annexation of Texas an act of war, and after border skirmishes, designed in part to provoke an attack upon US forces, President Polk asked for the war. Under Article I, Section 8 of the Constitution, only Congress can declare a war, and notwithstanding motives, that peculiarly quaint custom was still obeyed in the 19th century.
In the fighting that ensued, the mostly volunteer US military secured control of Mexico after a series of successful, if mutually bloody, battles. Presaging the Civil War in many respects, the conflict was populated by personalities including future doomed POTUS Zachary Taylor, Gen. Winfield Scott, Uly Grant, Bobby Lee and Lincoln himself. With Treaty of Guadalupe Hidalgo signing on February 2, 1848, it was all over.
In the first large-scale success of a US military force on foreign soil, Mexico received little more than $18 million in compensation. On the real estate end, the pact set a border between Texas and Mexico, and ceded California, Nevada, Utah, New Mexico, most of Arizona and Colorado, and parts of Oklahoma, Kansas, and Wyoming to the United States. The total territorial size of Mexico was reduced by half.
In today’s terms, those 10 states above account for 136 electoral votes, more than half of the votes needed to secure a win in a presidential election. While the war’s outcome seemed like a bonanza for the United States, the acquisition of so much territory with the issue of slavery still unresolved continued the course heading straight for the Civil War in 1861.
With the Missouri Compromise of 1850, attempted appeasement of Southern concerns about the shifting balances on slavery only served to compromise the integrity and decency of the US; the die was cast as the nation ambled inexorably toward Civil War.