Marcus Aurelius, the last of the Antonines, had to resist the invasion of several barbarous peoples of Germany who had crossed the Danube on the ice and had penetrated even to Aquileia, in the north of Italy. In order to enroll a sufficient army he had to enlist slaves and barbarians (172). The Germans retreated, but while Marcus was occupied with a general uprising in Syria, they renewed their attacks on the empire, and the emperor died on the banks of the Danube (180). This was the end of conquest. IMPERIAL ADMINISTRATION =Extent of the Empire in the Second Century.=--The Roman emperors were but little bent on conquest. But to occupy their army and to secure frontiers which might be easily defended, they continued to conquer barbarian peoples for more than a century. When the course of conquest was finally arrested after Trajan, the empire extended over all the south of Europe, all the north of Africa and the west of Asia; it was limited only by natural frontiers--the ocean to the west; the mountains of Scotland, the Rhine, the Danube, and the Caucasus to the north; the deserts of the Euphrates and of Arabia to the east; the cataracts of the Nile and the great desert to the south. The empire, therefore, embraced the countries which now constitute England, Spain, Italy, France, Belgium, Switzerland, Bavaria, Austria, Hungary, European Turkey, Morocco, Algiers, Tunis, Egypt, Syria, Palestine, and Asiatic Turkey. It was more than double the extent of the empire of Alexander. This immense territory was subdivided into forty-eight provinces,[147] unequal in size, but the majority of them very large. Thus Gaul from the Pyrenees to the Rhine formed but seven provinces. =The Permanent Army.=--In the provinces of the interior there was no Roman army, for the peoples of the empire had no desire to revolt. It was on the frontier that the empire had its enemies, foreigners always ready to invade: behind the Rhine and the Danube the barbarian Germans; behind the sands of Africa the nomads of the desert; behind the Euphrates the Persian army. On this frontier which was constantly threatened it was necessary to have soldiers always in readiness. Augustus had understood this, and so created a permanent army. The soldiers of the empire were no longer proprietors transferred from their fields to serve during a few campaigns, but poor men who made war a profession. They enlisted for sixteen or twenty years and often reënlisted. There were, then, thirty legions of citizens--that is, 180,000 legionaries, and, according to Roman usage, a slightly larger number of auxiliaries--in all about 400,000 men. This number was small for so large a territory.