Considering a couple of other threads in this forum _
America grabs even more of Mexico -- a LOT more -- than happened IOTL. Faced with massive increases in federal taxation to fund the army that's needed to maintain control there, the businessmen of New England and New York push/lead their states into seccession... and of course the US army is too deeply entangled in the new territories that were formerly parts of Mexico for the federal government to stop them by force, even if it wants to do so.