At the end of the fourth century the Roman Empire still comprised the entire basin of the Mediterranean. In Europe its continental limits were the Rhine and the Danube; in Asia, an undefined frontier, modified constantly by wars with the Armenians and Persians, followed the eastern slope of the Pontus Euxinus (Black Sea) to the foot of the Caucasus Mountains and extended into Armenia around Lake Van, thence in an almost straight line to the Red Sea, crossing the Tigris below Tigranocerta, and the Euphrates at its junction with the Chaboras at Circesium. On the south, Egypt up to and beyond the first cataract, and the northern slope of Africa, with Cyrenaica, Tripolitania, and Mauritania, belonged to Rome, which possessed in the valley of the Nile and in the modern Tunis the wheat granaries that supplied the hungry people of the two capitals. On the west the Atlantic Ocean formed the horizon of the ancients, who imagined beyond it the mysterious land of the blessed ones. On the north the island of Britannia belonged to the Empire, with the exception of the mountainous region of Caledonia, which retained its independence, as did Hibernia, or Ireland...