The expedition north of that point seems to have been either of the nature of a reconnaissance in force, on the results of which the king decided against further advance and attempt at conquest, or a display intended to strike awe into the tribes beyond the newly won territory. Whether any attempt at conquest in these northern regions formed part of the original programme may be doubted: one thing is certain, that the expedition in the form it was made was not anything of the kind.
It is noticeable that at this period the great river formed a very marked and striking ethnographical boundary between the tribes of Thracian stock and the Scyths of Europe. It has already been pointed out that Darius had within his dominions in Asia Minor—mainly in the region of Bithynia—people of Thracian origin. May he not have undertaken the expedition with intent to minimize the chances of disturbance on the weak imperial frontier formed by the Propontis and its two straits by subduing the free kinsmen of his Thracian subjects? It is a precaution such as the rulers of empires at every time in history and in every part of the world have frequently had to take. It may be, then, that Darius was in search of an ethnic frontier, and when he had found it on the Ister he sought not for further conquest, but confined himself to measures intended to secure what he had won.
The Ethnic Frontier.
It is perhaps unnecessary to insist on the importance of an ethnic frontier. It is, of course, frequently coincident with a physical frontier. Where such a coincidence exists, a “scientific” frontier is attained. But in cases where the two are not coincident, history has shown again and again that the ethnic is the more “scientific” of the two.
Regarding the question from another point of view,—it is infinitely more dangerous for a state to have on its borders a race who have kin immediately beyond the border in the enjoyment of a form of government which assures them their independence, or in which they are paramount, than to have on its frontier a race alien to their neighbours beyond the border.
The first of these alternatives is an almost inevitable source of trouble and danger to the state in which the circumstances exist. The danger is twofold. It may arise from the desire of those within the frontier to attain to the position enjoyed by their kinsmen outside it, or from the sympathy of the latter with those of their race who are in subjection to aliens.
Such considerations are not the mere abstractions of political philosophy, but are plain, practical questions which have called for practical solution from men to whom the very idea of political philosophy may never have been presented. They appear in actual history in a concrete form, and are therefore hard to recognize amid the numerous factors which are involved in the complicity of human action. ETHNIC FRONTIERS. The very use of such modern terms as “scientific” or “ethnic” frontier in reference to policies of ancient date has a tendency to create the idea that the person using them is mistakenly attributing to the past conceptions which are essentially of modern date. But are such conceptions of modern date? Is it not rather the form in which they are expressed which is of recent creation? The very creation of such a form of expression indicates, indeed, that the conception itself has become clearer in course of time; it rarely, if ever, indicates the actual birth of the conception. It is quite possible to grossly exaggerate the difference between the nature of the problems presented to the rulers of empire in the sixth century before Christ, and those which call for solution at the present time.
It is not difficult to illustrate from both ancient and modern history:—
(1) The dangers of a frontier which is not ethnic;
(2) The appreciation of this practical fact by those who have had to face the circumstances.