What possible US interest could be served by attacking Mexico? Mexico is an ally of the United States and attacking them would signal to the world that we have completely lost our minds. We would gain nothing from an attack and every single country in the world would be justified in condemning such an action.
Warring with Mexico is pointless and could not succeed at getting them to do anything different. The US citizens are fed up with war and anymore involvement in wars, especially with a border country. War will just increase the number of illegal immigrants into this country and the Mexican drug cartels will just supply our troops with drugs, like in Viet Nam. Mexico is an ally we have to keep an eye on, not make war with.