The main reason we didn't annex Mexico

I always thought the main reason the U.S. did not annex all of Mexico after the Mexican War was because of Northern opposition to creating more slave states.  That's why Henry David Thoreau went to jail–he thought the war was being fought to expand slavery.  He was right, of course, but that is not the whole story.

According to Peter Guardino (The Dead March, Harvard U. Press, 2017), Americans were fearful of incorporating Mexico into the U.S. because Mexicans were Catholic and of an inferior race.  California, Arizona, and New Mexico, which were annexed, contained relatively few Mexican nationals.  Mexico proper, on the other hand, contained millions of people then regarded in the U.S. as racially inferior.  This was also a period of intense anti-Catholic opinion.  The Mexican War occurred along with the rise of the Know Nothings, a political party based on opposing Catholic immigration.

Much of Trump's anti-Mexican rants and prejudices against Mexicans would have been right at home in 1846-48.  Bigotry doesn't change all that much.