Why did most Northerners tend to oppose the war with Mexico in the 1840s.

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.