May 12, 1846
On May 12, 1846, the United States Senate voted 40 to 2 to go to war with Mexico. President James K. Polk had accused Mexican troops of having attacked Americans on U.S. soil, north of the Rio Grande. But Mexico claimed this land as its own territory and accused the American military of having invaded. Texas senator Sam Houston argued that Texas had been warring with Mexico for a decade over their disputed border, and that once the United States had annexed Texas in 1845, it had inherited the conflict.
The House had already adopted the war resolution by a similarly lopsided margin. Despite this seemingly overwhelming support, the vote in the Senate masked great uneasiness and deep partisan divisions over the war.
Northern Whigs feared that war with Mexico would result in the United States gaining new territories in the southwest, which would encourage the expansion of slavery. At the same time, South Carolina Democratic senator John C. Calhoun worried that reopening the divisive issue of slavery in the territories would encourage more anti-slavery agitation.
Northern Whigs also questioned the need for war, but they remembered how New England’s opposition to the War of 1812 had destroyed the old Federalist Party, and they were anxious to avoid a similar political disaster. That left Senator Calhoun to lead the opposition to the war. Calhoun first tried to slow down the momentum by having the president’s message divided in two and referred to the Foreign Relations Committee and the Military Affairs Committee. The Senate agreed to his proposal, but declined to submit the actual war resolution to committee. The Senate as a whole debated that resolution.
The Northern Whigs responded by offering an amendment that would have limited the American role to “repelling the invasion.” That amendment failed by a vote of 20 to 26, revealing the real split within the Senate. Once that amendment failed, however, most of the Whigs switched their vote to favor the declaration of war. Senator Calhoun abstained.
Victory in the Mexican War gave the United States a vast new territory that stretched from California to New Mexico. As Calhoun had feared, the question of slavery in these territories quickly developed into an angry wedge between the North and South. It was during the summer of 1846 that Pennsylvania representative David Wilmot proposed a proviso to an appropriations bill, decreeing that slavery should never be permitted in any territory won from Mexico. The Wilmot Proviso ignited a heated political debate that affected both political parties, shaking their political coalitions and intensifying the sectionalism that would soon lead to the American Civil War.