Immigration, whether legal or illegal, will not usher in a global government. Fear, cheerleading, and egos will. Divide and conquer. GBRK is right about the Posse Comitatus Act. As long as the military refuses unlawful acts by the president, democracy will survive. The day the military does the bidding of a president without question is the day we all lose. It might still take a few decades but the seeds are planted. We already live in a global economy so the infrastructure is already in place.
The gop believes in small, limited government. That only gives power to the lobbyists, wealthy people, and corporations. Which will open the door to oligarchy and authoritarianism. Democracy and diversity are keeping "global government" in check. As long as people have the ability to fight for their rights; elections are free, fair and voting is easy; the press remains free and is not labeled as the "enemy of the people"; and people can manage to stop being scared of everyone that's different we will be alright.