Gallienus, fearing a revolt of the Senate, excluded senators from the army. As martial material no longer grew in Italy, this decree completed the military decline of the peninsula. The rise of provincial and mercenary armies, the overthrow of the Praetorian Guard by Septimius Severus, the emergence of provincial generals, and their capture of the imperial throne, destroyed the leadership, even the independence, of Italy long before the fall of the Empire in the West. The armies of Rome were no longer Roman armies; they were composed chiefly of provincials, largely of barbarians; they fought not for their altars and their homes, but for their wages, their donatives, and their loot. They attacked and plundered the cities of the Empire with more relish than they showed in facing the enemy; most of them were the sons of peasants who hated the rich and the cities as exploiters of the poor and the countryside; and as civil strife provided opportunity, they sacked such towns with a thoroughness that left little for alien barbarism to destroy. When military problems became more important than internal affairs, cities near the frontiers were made the seats of government; Rome became a theater for triumphs, a show place of imperial architecture, a museum of political antiquities and forms. The multiplication of capitals and the division of power broke down the unity of administration. The Empire, grown too vast for its statesmen to rule or its armies to defend, began to disintegrate. Left to protect themselves unaided against the Germans and the Scots, Gaul and Britain chose their own imperatores, and made them sovereign; Palmyra seceded under Zenobia, and soon Spain and Africa would yield almost unresisting to barbarian conquest. In the reign of Gallienus thirty generals governed thirty regions of the Empire in practical independence of the central power. In this awful drama of a great state breaking into pieces, the internal causes were the unseen protagonists; the invading barbarians merely entered where weakness had opened the door, and where the failure of biological, moral, economic, and political statesmanship had left the stage to chaos, despondency, and decay.

Externally the fall of the Western Roman Empire was hastened by the expansion and migration of the Hsiung-nu, or Huns, in northwestern Asia. Defeated in their eastward advance by Chinese armies and the Chinese Wall, they turned westward, and about A.D. 355 reached the Volga and the Oxus. Their pressure forced the Sarmatians of Russia to move into the Balkans; the Goths, so harassed, moved again upon the Roman frontiers. They were admitted across the Danube to settle in Moesia (376); maltreated there by Roman officials, they revolted, defeated a large Roman army at Adrianople (378), and for a time threatened Constantinople. In 400 Alaric led the Visigoths over the Alps into Italy, and in 410 they took and sacked Rome. In 429 Gaiseric led the Vandals to the conquest of Spain and Africa, and in 455 they took and sacked Rome. In 451 Attila led the Huns in an attack upon Gaul and Italy; he was defeated at Chalons, but overran Lombardy. In 472 a Pannonian general, Orestes, made his son emperor under the name of Romulus Augustulus. Four years later the barbarian mercenaries who dominated the Roman army deposed this “little Augustus,” and named their leader Odoacer king of Italy. Odoacer recognized the supremacy of the Roman emperor at Constantinople, and was accepted by him as a vassal king. The Roman Empire in the East would go on until 1453; in the West it had come to an end.