Most Northerners opposed the War with Mexico as they considered it an insidious attempt to extend slavery into new areas once those areas became part of the United States. John C. Calhoun not a Northerner, obviously, but aware of the pending debate, called land gained from Mexico
the forbidden fruit; the penalty of eating it would be to subject our institutions to political death.
Ralph Waldo Emerson commented:
The United States may conquer Mexico; but it will be as the man swallows the arsenic.
David Wilmot, a freshman Congressman from Pennsylvania proposed the Wilmot Proviso, which would have precluded slavery from any territory gained during the war. His proviso did not pass, but did bring the debate over the extension of slavery into focus. John C. Calhoun countered with the famous "Calhoun Resolutions" which stated that the Federal Government had no right to prevent any citizen from taking slaves into the territory. Senator Thomas Hart Benton commented after this that Calhoun and Wilmot had formed a share of shears;; neither one by itself could accomplish anything, but together they could sever the Union. His words were prophetic.