Caliphate

What happens when someday in our lifetime some group of radical muslims declare a fellow radical to be the Caliphate? By their reasoning, this guy will be the holy ruler over Islam and all subjects loyal to him. I imagine this will pose problems for sovereignty and our whole nation-state system.

And this is really what this entire Islamic Fundamentalist movement is all about, isn't it? They're just prepping for the day to restore power to one central, Islamic body, which will rule over all Muslims. Will non-radical muslims recognize this person? Will they be subjects? I see a movie.