The Roman Empire

A monarchy assembled entirely out of republican offices, described in republican language, with no name, no founding law, and no rule for what happened when its holder died.

A position with no rule of succession had to be filled somehow, and the improvisations that filled it show exactly who was deciding.

A quarter of a million professional soldiers on fixed terms, stationed where the emperor could reach them and nowhere near Italy, was the empire's largest institution and its only universal one.

A coinage, a census and a handful of taxes had to produce the army's wages every year, and the arithmetic explains both why the burden was light and why there was no margin.

A few hundred senior officials governed sixty million people by handing almost every task of government to about two thousand self-governing cities and the local rich who ran them.

Citizenship began as the privilege of one Italian city and ended as a status held by every free inhabitant of the empire, which is why the men who ruled it increasingly came from the places it had conquered.

Rome had grown for four centuries on the profits of the next war, and within a generation of Augustus the growth stopped, because the land beyond the frontier was not worth what it cost to take.

An economy with no statistics can still be measured, from a hill of broken jars, a catalogue of shipwrecks and the lead trapped in Greenland ice, provided the limits of each proxy are stated as clearly as the result.

Roman religion asked for correct performance rather than belief, which made it endlessly hospitable to new gods and completely unequipped for a group that would not sacrifice to any of them.

Between 235 and 284 the empire got through roughly twenty-six emperors, lost one to capture by a Persian king, split into three states and destroyed its own silver coinage, and then put itself back together.

Four rulers, a hundred provinces, an army paid in requisitioned goods and a tax assessed on every field and every head: the answer to the third century was a state twice the size and far harder to ignore.

A usurper who won eighteen years of civil war founded a new capital, struck a gold coin that lasted seven centuries, and attached the empire to a church, which put a second authority inside the Roman state.

Once the state adopted a church it had to say what that church believed, which turned theology into imperial business, made bishops into civic powers, and produced the first Roman emperor ever compelled to do public penance.

The groups that ended up ruling the western provinces were small, were mostly admitted rather than invading, and had been shaped for centuries by contact with the empire they took over.

The western empire lost the provinces that paid for its army, and an army that cannot be paid becomes somebody else's, which is a better account of the fifth century than any invasion.

Not the name, not the law, not the church and not the eastern state: what ended in the west was a tax-funded army and the complex economy that only that kind of state could hold together.