The division after the Mexican American War began with the introduction of the Wilmot Proviso which argued that slavery would not be permitted in the territory gained from the war. The proviso was added to an appropriations bill, and was similar to the language of the old Northwest Ordinances which had forbidden slavery in the old Northwest. Wilmot, a freshman Congressman from Pennsylvania made a long speech in support of the proviso during which he exclaimed:
God forbid that we should be the means of planting this institution upon it."
The proviso came up for a vote twice, was passed in the House, but defeated in the Senate. It had long been believed that the slavery issue had been settled by the Missouri Compromise; in fact President James K. Polk had proposed extending the Compromise line into the new territories. The Wilmot proviso only ignited the debate anew. In response to the proviso, John C. Calhoun offered the Calhoun Resolutions which argued that the new territories were the common possession of the several states, and the provisions of the 5th Amendment against taking property without just compensation applied; therefore slavery could not legally be prohibited. Although Calhoun's resolutions never came for a vote, his position was soon adopted by Southern representatives in the ensuing debates over slavery.
The situation was perhaps best described by Senator Thomas Hart Benton who said that Wilmot's proviso and Calhoun's resolutions were like a pair of shears. One alone could do nothing; but together they could sever the ties of the union. His words were prophetic.
The division over the Mexican-American War can be seen as a prelude to the Civil War because it pitted Southerners against Northerners over the issue of slavery.
Before the Mexican-American War, the US was evenly divided between slave states and free states. As the war approached, it was seen by many in the North as an attempt to gain more territory for the slave states. Because of this, many Northerners (like Abraham Lincoln, who was in Congress at that point) opposed the war. Thus, the divisions over the Mexican-American War (like the Civil War itself) were North-South divisions over the issue of slavery.