The outward and visible sign of the Empire, which above everything else distinguished it from the world which surrounded it, as an island is surrounded by an unmapped sea, was the Pax Romana. This was domestic as well as political. It ensured a complete and absolute order, the condition of civilisation, and, established through many generations, it seemed immutable and unbreakable. Along with it went a conception of law and of property more fundamental than anything we are now able to appreciate, while free exchange was assured by a complete system of communication and admirable roads. There is indeed scarcely anything that is really fundamental in our lives and in our politics that was not there created. It was there our religion, the soul of Europe, was born and little by little became the energy and the cause of all that undying but changeful principle of life and freedom which rightly understood is Europe. Our ideas of justice, our ideas of law, our conception of human dignity and the structure of our society were there conceived, and with such force that while we endure they can never die. In truth, the Empire which it had taken more than a millennium to build was the most successful and perhaps the most beneficent experiment in universal government that has ever been made. The Empire fell. Why? We cannot answer that question. The causes of such a catastrophe, spiritual and material, are for the most part hidden from us in the darkness that followed the catastrophe, in which civilisation in the West all but perished. All we can do is to note that the administration of this great State became so expensive that when Alaric came over the Alps in 401 it was probably already bankrupt and in consequence the population was declining; and that the military problem before the Empire, the defence of its frontiers against the outer welter of barbarism, was so expensive and so naturally insecure that it was difficult to ensure and impossible with due economy. Finally we ought to be sure that though the Empire decayed and fell, it was not overthrown by the Barbarians. As in this book we are concerned not indeed with the Barbarian invasions as a whole but nevertheless with the most frightful and perhaps the most destructive among them, we shall do well to consider more particularly here for a moment one of the causes of that fall, though not the chief one as we have said; the insecurity of the frontiers, namely, and the problem this proposed which the Empire was, alas, unable to solve. The Empire was confined on the west by the ocean, on the south by the desert of Africa, on the east by the Caspian Sea and the Persian Gulf, on the north by the Rhine and the Danube, the Black Sea and the Caucasus. It was that northern frontier which was a fundamental weakness and which at least from the middle of the third century continually occupied the mind of the Roman administration. How to hold it?