In order to understand what happened we must first of all clearly represent to ourselves the fact that the structure upon which our united civilization had in its first five centuries reposed, was the _Roman Army_. By which I do not mean that the number of soldiers was very large compared with the civilian population, but that the organ which was vital in the State, the thing that really counted, the institution upon which men’s minds turned, and which they thought of as the foundation of all, was the military institution. The original city-state of the Mediterranean broke down a little before the beginning of our era. When (as always ultimately happens in a complex civilization of many millions) self-government had broken down, and when it was necessary, after the desperate faction fights which that breakdown had produced, to establish a strong centre of authority, the obvious and, as it were, necessary person to exercise that authority (in a State constituted as was the Roman State) was the Commander-in-Chief of the army; all that the word “Emperor”—the Latin word _Imperator_—means, is a commander-in-chief. It was the Army which made and unmade Emperors; it was the Army which designed and ordered and even helped to construct the great roads of the Empire. It was in connection with the needs of the Army that those roads were traced. It was the Army which secured (very easily, for peace was popular) the civil order of the vast organism. It was the Army especially which guarded its frontiers against the uncivilized world without; upon the edge of the Sahara and of the Arabian desert; upon the edge of the Scotch mountains; upon the edge of the poor, wild lands between the Rhine and Elbe. On those frontiers the garrisons made a sort of wall within which wealth and right living could accumulate, outside which small and impoverished bodies of men destitute of the arts (notably of writing) save in so far as they rudely copied the Romans or were permeated by adventurous Roman commerce, lived under conditions which, in the Celtic hills, we can partially appreciate from the analogy of ancient Gaul and from tenacious legends, but of which in the German and Slavonic sand-plains, marshes and woods we know hardly anything at all. Now this main instrument, the Roman Army—the instrument, remember, which not only preserved civil functions, but actually created the master of all civic functions, the Government—went through three very clear stages of change in the first four centuries of the Christian era—up to the year A.D. 400 or so. And it is the transformation of the Roman Army during the first four centuries which explains the otherwise inexplicable change in society just afterwards, in the fifth and sixth centuries—that is, from 400 to 600 A.D. The turn from the full civilization of Rome to the beginning of the Dark Ages.