The modern state whose youth was spent in strife has entered its turbulent old age. Now, as then, the old order is marshalling its forces of arms and ideas to defend itself against the impact of the new. Just as the medieval church-state was once attacked by the protagonists of the national state, so this latter is now attacked in turn by those seeking to break the bonds of a narrow nationalism. Beneath the struggle lies the ineradicable craving for fulfilment. Where once it was expressed in terms of unity under the headship of the Vicar of Christ, since the Reformation it has taken the form of unity within a sovereign state. And where once the claims of the Pope were subjected to a merciless criticism, now the claims of the sovereign are subjected to an equally relentless stricture. The opposition is no new thing, for the history of political thought in the last five centuries revolves around the attempt to impose restrictions upon the sovereign authority of the state. But it is significant that not until recently has the argument taken the form of a denial of sovereignty. This means, of course, that the assumptions underlying the political system are being challenged. Nor is it only ideas; it is also the social and economic institutions which are being poured into the crucible of criticism. The result of the withdrawal of reverence is to illuminate the nature of sovereignty, thereby enabling us to clarify its functions and to evaluate its worth.