On the whole, however, it is astonishing, as I have observed before, how small a proportion the additions made to International Law since Grotius's day bear to the ingredients which have been simply taken from the most ancient stratum of the Roman Jus Gentium. Acquisition of territory has always been the great spur of national ambition, and the rules which govern this acquisition, together with the rules which moderate the wars in which it too frequently results, are merely transcribed from the part of the Roman law which treats of the modes of acquiring property _jure gentium_. These modes of acquisition were obtained by the elder jurisconsults, as I have attempted to explain, by abstracting a common ingredient from the usages observed to prevail among the various tribes surrounding Rome; and, having been classed on account of their origin in the "law common to all nations," they were thought by the later lawyers to fit in, on the score of their simplicity, with the more recent conception of a Law Natural. They thus made their way into the modern Law of Nations, and the result is that those parts of the international system which refer to _dominion_, its nature, its limitations, the modes of acquiring and securing it, are pure Roman Property Law--so much, that is to say, of the Roman Law of Property as the Antonine jurisconsults imagined to exhibit a certain congruity with the natural state. In order that these chapters of International Law may be capable of application, it is necessary that sovereigns should be related to each other like the members of a group of Roman proprietors. This is another of the postulates which lie at the threshold of the International Code, and it is also one which could not possibly have been subscribed to during the first centuries of modern European history. It is resolvable into the double proposition that "sovereignty is territorial," _i.e._ that it is always associated with the proprietorship of a limited portion of the earth's surface, and that "sovereigns _inter se_ are to be deemed not _paramount_, but _absolute_, owners of the state's territory."
Many contemporary writers on International Law tacitly assume that the doctrines of their system, founded on principles of equity and common sense, were capable of being readily reasoned out in every stage of modern civilisation. But this assumption, while it conceals some real defects of the international theory, is altogether untenable, so far as regards a large part of modern history. It is not true that the authority of the Jus Gentium in the concerns of nations was always uncontradicted; on the contrary, it had to struggle long against the claims of several competing systems. It is again not true that the territorial character of sovereignty was always recognised, for long after the dissolution of the Roman dominion the minds of men were under the empire of ideas irreconcileable with such a conception. An old order of things, and of views founded on it, had to decay--a new Europe, and an apparatus of new notions congenial to it, had to spring up--before two of the chiefest postulates of International Law could be universally conceded.