And with decay within came danger from without.

For centuries past the Roman frontier had held back the barbaric world beyond it, the Parthian of the Euphrates, the Numidian of the African desert, the German of the Danube or the Rhine.

In Britain a wall drawn from Newcastle to Carlisle bridled the British tribes, the Picts as they were called, who had been sheltered from Roman conquest by the fastnesses of the Highlands.

It was this mass of savage barbarism which broke upon the Empire as it sank into decay.

In its western dominions the triumph of these assailants was complete.