The empire which it had taken more than a millenium to build, which was the most noble and perhaps the most beneficient experiment in government that has ever been made, was in obvious economic and administrative decay by the middle of the fourth century. Christianity perhaps was already undermining the servile state, which in its effort of self-preservation adopted an economic system hopelessly at variance with the facts of the situation; while the weakness of its frontiers offered a military problem which the empire was unable to face. Diocletian had attempted to solve it by dividing the empire, but the division he made was rather racial that strategic, for under it the two parts of the empire, East and West, met on the Danube. The eastern part, by force of geography, was inclined to an Asiatic point of view and to the neglect of the Danube; the western was by no means strong enough either financially or militarily to hold that tremendous line.
We read, in the letters of S. Ambrose among others, of the decay of the great cities of Cisalpine Gaul, of the failure of agriculture in that rich countryside, of the poverty and misery that were everywhere falling upon that great state. It is possible that in the general weakening of administrative power even the roads, the canals, the whole system of communications were allowed to become less perfect than they had been; everywhere there was a retreat. The frontiers were no longer inviolate, and it is probable that in the general decay the port of Classis, the city of Ravenna, suffered not less than their neighbours.
[Footnote 1: See S. Ambrose, Ep. 39, written in 388, quoted by Muratori, Dissertazioni, vol. i. 21. “De Bonomensi veniens Urbe, a tergo Claternam, ipsam Bononiam, Mutinam, Regium derelinquebas; in dextera erat Brixillum; a fronte occurrebat Placentia.... Te igitur semirutarum Urbium cadavera, terrarumque sub eodem conspectu exposita funera non te admonent....”]
Indeed already in 306 it is rather as a refuge than as a great and active naval base that Ravenna appears to us, when Severus, destitute of force, “retired or rather fled” thither from the pursuit of Maximian. He flung himself into Ravenna because it was impregnable and because he expected reinforcements from Illyricum and the East, but though he held the sea with a powerful fleet he made no use of it, and the emissaries of Maximian easily persuaded him to surrender. Already perhaps, a century later, when Honorius retired from Milan on the approach of Alaric and the first of those barbarian invasions which broke up the decaying western empire had penetrated into Cisalpine Gaul, the great works of Augustus and Trajan at Ravenna, the canals, the mighty Fossa, and the port itself had fallen into a sort of decay which the fifth century was to complete, till that marvellous city, once the base of the eastern fleet and one of the great naval ports of the world, became just a decaying citadel engulfed in the marshes, impregnable it is true, but for barbarian reasons, lost in the fogs and the miasma of her shallow and undredged lagoons.