A longer excerpt from the The Lessons of History on the decline of civilizations:
When the group or a civilization declines, it is through no mystic limitation of a corporate life, but through the failure of its political or intellectual leaders to meet the challenges of change.
The challenges may come from a dozen sources, and may by repetition or combination rise to a destructive intensity. Rainfall or oases may fail and leave the earth parched to sterility. The soil may be exhausted by incompetent husbandry or improvident usage. The replacement of free with slave labor may reduce the incentives to production, leaving lands untilled and cities unfed. A change in the instruments or routes of trade—as by the conquest of the ocean or the air—may leave old centers of civilization becalmed and decadent, like Pisa or Venice after 1492. Taxes may mount to the point of discouraging capital investment and productive stimulus. Foreign markets and materials may be lost to more enterprising competition; excess of imports over exports may drain precious metal from domestic reserves. The concentration of wealth may disrupt the nation in class or race war. The concentration of population and poverty in great cities may compel a government to choose between enfeebling the economy with a dole and running the risk of riot and revolution.
Since inequality grows in an expanding economy, a society may find itself divided between a cultured minority and a majority of men and women too unfortunate by nature or circumstance to inherit or develop standards of excellence and taste. As this majority grows it acts as a cultural drag upon the minority; its ways of speech, dress, recreation, feeling, judgment, and thought spread upward, and internal barbarization by the majority is part of the price that the minority pays for its control of educational and economic opportunity.
As education spreads, theologies lose credence, and receive an external conformity without influence upon conduct or hope. Life and ideas become increasingly secular, ignoring supernatural explanations and fears. The moral code loses aura and force as its human origin is revealed, and as divine surveillance and sanctions are removed. In ancient Greece the philosophers destroyed the old faith among the educated classes; in many nations of modern Europe the philosophers achieved similar results. Protagoras became Voltaire, Diogenes Rousseau, Democritus Hobbes, Plato Kant, Thrasymachus Nietzsche, Aristotle Spencer, Epicurus Diderot. In antiquity and modernity alike, analytical thought dissolved the religion that had buttressed the moral code. New religions came, but they were divorced from the ruling classes, and gave no service to the state. An age of weary skepticism and epicureanism followed the triumph of rationalism over mythology in the last century before Christianity, and follows a similar victory today in the first century after Christianity.
Caught in the relaxing interval between one moral code and the next, an unmoored generation surrenders itself to luxury, corruption, and a restless disorder of family and morals, in all but a remnant clinging desperately to old restraints and ways. Few souls feel any longer that "it is beautiful and honorable to die for one's country." A failure of leadership may allow a state to weaken itself with internal strife. At the end of the process a decisive defeat in war may bring a final blow, or barbarian invasion from without may combine with barbarism welling up from within to bring the civilization to a close.
Is this a depressing picture? Not quite. Life has no inherent claim to eternity, whether in individuals or in states. Death is natural, and if it comes in due time it is forgivable and useful, and the mature mind will take no offense from its coming. But do civilizations die? Again, not quite. Greek civilization is not really dead; only its frame is gone and its habitat has changed and spread; it survives in the memory of the race, and in such abundance that no one life, however full and long, could absorb it all. Homer has more readers now than in his own day and land. The Greek poets and philosophers are in every library and college; at this moment Plato is being studied by a hundred thousand discoverers of the "dear delight" of philosophy overspreading life with understanding thought. This selective survival of creative minds is the most real and beneficent of immortalities.
Nations die. Old regions grow arid, or suffer other change. Resilient man picks up his tools and his arts, and moves on, taking his memories with him. If education has deepened and broadened those memories, civilization migrates with him, and builds somewhere another home. In the new land he need not begin entirely anew, nor make his way without friendly aid; communication and transport bind him, as in a nourishing placenta, with his mother country. Rome imported Greek civilization and transmitted it to Western Europe; America profited from European civilization and prepares to pass it on, with a technique of transmission never equaled before.
Civilizations are the generations of the racial soul. As life overrides death with reproduction, so an aging culture hands its patrimony down to its heirs across the years and the seas. Even as these lines are being written, commerce and print, wires and waves and invisible Mercuries of the air are binding nations and civilizations together, preserving for all what each has given to the heritage of mankind.