How can we tell if a populace is unified or not? If Canada and the US merged, would it be more "unified"? Would the world population be more "unified" under a w

How can we tell if a populace is unified or not? If Canada and the US merged, would it be more "unified"? Would the world population be more "unified" under a world government? If not, what makes the existing governments more legitimate than those fake "unification dreams"?