Edward Gibbon’s The History of the Decline and Fall of the Roman Empire is, obviously, not light reading. Still, it’s more readable than you might expect. More rational, too — as a son of the Enlightenment, Gibbon is critical of religion; indeed, he’s one of the earliest real critics of Christianity, and wrote at a time when to do so still carried real personal risk. At the same time, he predates Marx, Freud, and other modern thinkers. Modern historians largely see him as old-fashioned, and his views as simplistic. In other words, he stands against both religious dogma and modern nihilism.
I disagree with the moderns. Though Gibbon lacks some modern knowledge (such as that gained from the last two centuries of archaeology), he is wrongly labeled as simplistic for setting out a grand integration of Roman history. Modern history tends to see historical events as being too complex to be traced back to individual causes; instead, events must be explained by a wide range of overlapping factors. But causes and effects form a tree-like structure, and everything can be traced back to a small set of fundamental causes. Toyota understands this, and so does Gibbon.
One keystone of his theory is the classical notion of virtue: habits of thought and action that lead to success in reality. Vice, in contrast, leads to destruction and disaster. The Roman Republic grew strong by building a social system that rewarded and encouraged the virtuous. After it became an empire, the social order slowly degenerated, and philosophical, wise, courageous, and far-sighted emperors gradually became replaced by brutal, vain, cruel, and short-sighted rulers. Gibbon traces this trend through the centuries, and through many aspects of Roman society.
In the purer ages of the commonwealth, the use of arms was reserved for the ranks of citizens who had a country to love, a property to defend, and some share in enacting those laws, which it was their interest, as well as duty, to maintain. But in proportion as the public freedom was lost in extent of conquest, war was gradually improved into an art, and degraded into a trade. The legions themselves, even at the time when they were recruited into the most distant provinces, were supposed to consist of Roman citizens. That distinction was generally considered, either as a legal qualification, or as a proper recompense for the soldier; but a more serious regard was paid to the essential merit of age, strength and military stature. […] After every qualification of property has been laid aside, the armies of the Roman emperors were still commanded, for the most part, by officers of a liberal birth and education; but the common soldiers, like the mercenary troops of modern Europe, were drawn from the meanest, and very frequently from the most profligate, of mankind.
The public virtue which amongst he ancients was denominated patriotism, is derived from a strong sense in our own interest in the preservation and prosperity of the free government of which we are members. Such a sentiment, which had rendered the legions of the republic almost invincible, could make but a very feeble impression on the mercenary servants of a despotic prince; and it became necessary to supply that defect by other motives, of a different, but not less forcible nature; honour and religion. […] The golden eagle, which glittered in front of the legion, was the object of their fondest devotion; nor was it esteemed less impious, than it was ignominious*, to abandon that sacred ensign in the hour of danger.
* (Ignominious: very shameful; reproachful; dishonourable; infamous).
You can get a taste of the rest of the book here.