Justification, maybe. Of the moral kind.

I went back to Morris's Essay on the Modern State last night, and, althougth I'm not sure, I think I should be looking for the moral justification for the State (I.e. Why should the State exist? It should exist because it is a just form of social organisation. Of course, different ethical theories will disagree on what consititues justice). If I'm right here, then legitimacy should naturally flow out of a moral justification, and I shouldn't have to worry too much. Point being is that I'm looking for a moral reason for the State's existance.