But why did Rome fall? We have far too many answers. There is the intellectual answer: Montesquieu said that the Romans conquered the world with their republican principles, they changed their principles to fit an empire, and the new principles destroyed it. There is the moral answer: license, luxury, and sloth, a decline in character and in discipline. The Christian answer of Saint Augustine: sinful Rome fell to prepare for the triumph of the City of God. The rationalist answer of eighteenth-century freethinkers: Christianity, teaching nonresistance, otherworldliness, disarmed the Romans in the face of the barbarians. The political answer: Caesarism, loss of public spirit, the failure of the civil power to control the army. The social answer or answers: the war of the classes and the institution of slavery, which suppresses incentives toward change and progress. The economic answer: trade stagnation, low productivity, scarcity of gold and silver. The physical answer: soil depletion, deforestation, climatic change, drought. The pathological answer: plague and malaria, or even lead poisoning from cooking pots and water pipes. The genetic and racial answers: the dwindling of the old Roman stock through war and birth control and its mingling with Oriental and barbarian breeds. And the biological-cyclical-mystical answer: an empire is an organism, and like a living creature, it must pass through stages of growth, maturity, and decline, to death.
—Morris Bishop - “The Middle Ages”