Why did many americans criticize the mexican war? how did they see expansion as a threat to american liberties?

Northerners saw the Mexican-American War as a way for the South to try and gain control in the government. The North vs South only existed because it depends on how many of each state there was (slave or nonslave state). The North saw slavery as an abomination and that it went against the constitution " all men are created equal". But South thought that the bible was in their favor because it has passages supporting slavery. (sorry this is long)