Loss of army discipline
The History of the Decline and Fall of the Roman Empire, By Edward Gibbon
"It is the just and important observation of Vegetius, that the infantry was invariably covered with defensive armor, from the foundation of the city to the reign of the emperor Gratian. The relaxation of discipline, and the disuse of exercise, rendered the soldiers less able, and less willing, to support the fatigues of the service; they complained of the weight of the armor, which they seldom wore: and they successively obtained the permission of laying aside both their cuirasses and their helmets. The heavy weapons of their ancestors, the short sword, and the formidable pilum, which had subdued the world, insensibly dropped from their feeble hands. As the use of the shield is incompatible with that of the bow, they reluctantly marched into the field; condemned to suffer either the pain of wounds, or the ignominy of flight, and always disposed to prefer the more shameful alternative. The cavalry of the Goths, the Huns, and the Alani, had felt the benefits, and adopted the use, of defensive armor; and, as they excelled in the management of missile weapons, they easily overwhelmed the naked and trembling legions, whose heads and breasts were exposed, without defence, to the arrows of the barbarians. The loss of armies, the destruction of cities, and the dishonor of the Roman name, ineffectually solicited the successors of Gratian to restore the helmets and cuirasses of the infantry. The enervated soldiers abandoned their own, and the public defence; and the pusillanimous indolence may be considered as the immediate cause of the downfall of the empire." (Vol. III, pp. 271-272).