The assimilation of cities and states dominated by Mexican immigrants will accelerate. Americans will vote with with their feet (as they usually do) and we will end up with two (or more) de facto nations. I don't believe there will be a civil war to take back these regions, just a hardening of the boundaries between the two nations. An important indicator will be the infusion of Spanish into official life. For example, a key milestone will be reached when every televised speech by the President or other important official will be required (not by law, necessarily, but by practice) to have a Spanish translation (using SAP).