It may be doubted whether many students of history are aware that the demarcation of frontiers, of precise lines dividing the possessions of adjacent sovereignties and distinguishing their respective jurisdictions, is a practice of modern origin. At the present time it is the essential outcome of territorial disputes, it is the operation by which they are formally settled at the end of a war: it registers conquests and cessions; and occasionally it has been the result of pacific arbitration. Among compact and civilised nationalities an exterior frontier, thus carefully defined, remains, like the human skin, the most sensitive and irritable part of their corporate constitution. The slightest infringement of it by a neighbouring Power is instantly resented; to break through it violently is to be inflicting a wound which may draw blood; and even interference with any petty State that may lie between the frontiers of two great governments is regarded as a serious menace. The whole continent of Europe has now been laid out upon this system of strict delimitation. Yet it may be maintained that among the kingdoms of the ancient world no such exact and recognised distribution of territory existed; and, further, that up to a very recent period none of the great empires in Asia had any boundaries that could be traced on a map. Their landmarks were incessantly shifting forward or backward as their military strength rose or fell; and where their territories marched with some rough mountainous tract inhabited by warlike tribes, they were perpetually plagued by petty warfare on a zone of debateable land. On both sides some temporary intrusion upon or occupation of country held by a neighbour, which would now be the signal for mobilising an army, was treated as a trespass of small importance, to be resented and rectified at leisure. It is true that in earlier times the Romans marked off distinct frontiers, and guarded them by military posts; but their policy was to acknowledge no frontier power with equal rights, and their actual political jurisdiction usually extended far beyond their lines of defence, which were advanced or withdrawn as political or military considerations might require. In fact, the Roman empire, like the British empire in Asia, was a great organised State, surrounded, for the most part, by small and weak principalities, or by warlike tribal communities, and it grew by a natural process of inevitable expansion. The emperors were often reluctant to enlarge their possessions; but the raids and incursions of intractable barbarians, or the revolt of some protected chiefship, frequently left them no option but to conquer and annex. They soon found themselves compelled to overstep the limits of empire prescribed by the policy of Augustus, and to lay down an advanced frontier in the lands beyond the Rhine and the Danube.