Every Roman institution before Augustus assumed that the empire would keep getting bigger, and from the first century onwards it essentially stopped, which changed what the empire had to live on.
The previous lessons have described a state whose largest expense was a standing army, whose revenue came from taxing provinces, and whose citizenship spread outward through those provinces. All of that describes a machine running on a fixed input. Under the Republic the input had not been fixed: conquest brought indemnities, plunder, slaves and new tribute, and the earlier course on the Republic shows the wars of the second century BC paying for themselves several times over. This lesson asks when that ended, and why.
The year it stopped
In September of AD 9 the governor of Germany, Publius Quinctilius Varus, marched three legions and their auxiliaries through wooded country towards a reported rising, guided by a Roman-trained Cheruscan officer named Arminius who had arranged the report. Over three or four days the column was destroyed. Varus killed himself. The legions were the Seventeenth, Eighteenth and Nineteenth, and their numbers were never used again by the Roman army, which is as close to an official admission of disaster as Rome ever made.
The site was found in 1987 near Kalkriese, north of Osnabrück, by a British army officer with a metal detector, and the excavation has produced the shape of the ambush: a turf and timber wall built along a sand ridge, with the boggy ground of a moor on the other side, funnelling the column into a gap a few hundred metres wide. Among the finds are coins, and the coins date the site, because none of the many hundreds recovered is later than the issues Varus himself countermarked. Archaeology rarely confirms a literary account this cleanly.
Suetonius says Augustus beat his head against a door and called on Varus to give back the legions. Tacitus reports something more consequential: in the papers left at his death, Augustus advised that the empire be kept within its existing frontiers. Whether that was strategic wisdom or the rationalisation of an old man who had just lost an eighth of his army, his successors largely followed it.
What the advice meant in practice
Not that nothing moved. Claudius invaded Britain in 43, in part because he needed a victory to establish himself. Trajan took Dacia in two wars between 101 and 106, and then invaded Parthia, reaching the Persian Gulf in 116. But look at what happened next: Hadrian abandoned the whole Mesopotamian conquest on his accession in 117, and no emperor after Trajan added a province that lasted. Set against four centuries in which Rome had absorbed Italy, Spain, Gaul, Greece, Asia, Syria, Africa and Egypt, the imperial record is one significant permanent addition, Britain and Dacia, and Dacia was abandoned in the 270s.
The question is why. The usual answer, that Rome had reached its natural limits, is not much of an explanation, since the empire stopped at the Rhine rather than the Elbe, and neither is a natural line. The better answer is arithmetic.
The last conquest that paid
Dacia was rich in gold. The mines of the western Carpathians were worked by Rome immediately after the conquest, with imported miners from Dalmatia, and the wax tablets recording their contracts survive. The plunder from the war was enormous, and one late source, John the Lydian, quoting Trajan's own doctor Criton, gives a figure.
Example. Criton's figure as transmitted is 5,000,000 Roman pounds of gold and 10,000,000 of silver. Test it against the size of the Roman economy.
A Roman pound is 327 grams and the aureus was struck at 42 to the pound. Five million pounds of gold is therefore 210 million aurei, which at 25 denarii and 4 sesterces to the denarius is 21 billion sesterces. Against an imperial revenue near 800 million a year, that is twenty-six years of the entire income of the empire carried off from one province, and 1,635 tonnes of gold, which is a substantial share of all the gold mined in the ancient world. The figure is impossible.
Now apply the standard correction. Ancient numerals are corrupted in transmission more often than any other kind of text, and dividing by ten is the commonest repair, giving 500,000 pounds of gold and a million of silver. That is 2.1 billion sesterces, or about two and a half years of imperial revenue, and 163 tonnes of gold. Still spectacular, still the largest windfall of the imperial period, and now within the bounds of the possible. Notice how the two versions are treated differently: the impossible one is evidence that a big number was remembered, the emended one is a usable estimate, and neither should be quoted without saying which it is.
Now you. Trajan's triumph was followed by 123 days of games with a reported 10,000 gladiators and 11,000 animals, and the building of a new forum in Rome. What is the connection to the arithmetic above, and what did it fail to buy?
Answer
The connection is direct: the games, the forum, the column, the alimenta payments and a cash handout to the citizens of Rome were the Dacian gold being spent. Ten thousand gladiators over 123 days is about eighty appearing a day, which is a plausible schedule and an implausible expense for anything but a windfall, and Trajan's Forum is the largest single building project any emperor undertook. Spending plunder on display was what a Roman conqueror had always done, and the point of doing it was legitimacy.
What it did not buy was recurrence. Dacia was one province with an unusual endowment, and taking it did not make the next conquest more affordable; it made expectations higher. Trajan's Parthian war followed, and it produced no gold, no permanent province and a rebellion behind the lines, and his successor abandoned it within months of the funeral. The Dacian windfall is best read as the last of a Republican kind of event happening inside an imperial system that could no longer generate them.
Why the next province was not worth taking
The clearest ancient statement of the calculation is Strabo's, written under Augustus, about Britain. Some of the British chiefs, he says, have obtained friendship with Rome and the trade already pays customs duties on the crossing in both directions; to garrison the island would need at least one legion and some cavalry, the cost of which would have to come out of the tribute, so the revenue would be reduced by the expense of the occupation and the risks would increase. Appian, writing after the conquest had happened, says the Romans hold the better part of Britain and have no need of the rest, since even what they hold is not profitable.
Example. Turn Strabo's remark into a general rule and use it to explain the shape of the empire.
The rule is that a province is worth taking when the tax it will yield exceeds the permanent cost of the troops needed to hold it, and that both terms depend on what is already there. Tax yield depends on a settled agricultural surplus, cities that can assess and collect it, and a population used to paying somebody. Garrison cost depends on terrain and on how organised the resistance is.
Run the rule around the frontier and it predicts the map. Egypt, Syria and Asia had cities, records and a habit of taxation, and Rome took them and held them with very few troops. Germany beyond the Rhine had no cities, no coinage of its own, no scribes and a dispersed population in difficult country, so the tax yield is small and the garrison cost is permanent and high. Dacia was the exception that proves the rule, because gold in the ground substitutes for a taxable population. Britain sits at the margin and, on Appian's testimony, on the wrong side of it. The empire's boundary is not a natural line and not a strategic doctrine: it is roughly the contour where the expected revenue met the cost of the legions.
Now you. If the calculation was that clear, why did Claudius invade Britain in 43 anyway?
Answer
Because emperors were not maximising revenue. Claudius had been made emperor by the praetorian guard, had no military record of any kind in a system where military success was the standard justification for holding power, and needed a war he could win against an enemy famous enough for the victory to register in Rome. Britain was ideal: exotic, associated with Caesar, and reachable. He went out himself for sixteen days, was present for the taking of Camulodunum, and returned to a triumph and a triumphal arch.
The lesson is that the economic rule tells you which conquests are sustainable, not which are attempted. Political need explains the attempt; economics explains why Britain kept three legions for three centuries and never repaid them, why the northern third of the island was never held, and why nobody tried Ireland. Two different logics operate at once here, and an account that uses only one of them will get either the decisions or the outcomes wrong.
What the frontier actually was
With expansion over, the edges had to be marked, and from Hadrian onwards they were, in different forms. In Britain, a wall of stone running 117.5 km from Wallsend to Bowness, begun in 122, with a milecastle roughly every Roman mile and two turrets between each pair. In Upper Germany and Raetia, a palisade and later a wall and ditch running some 550 km with around 900 watchtowers behind it. Elsewhere the frontier was a river, the Rhine, the Danube and the Euphrates, patrolled by fleets.
Example. The garrison on and immediately behind Hadrian's Wall was on the order of 10,000 men. Work out what the wall could and could not do.
Ten thousand men over 117.5 km is about 85 per kilometre, and most of them were in forts rather than standing on it. That is enough to watch, to challenge, to stop small parties and to raise an alarm, and nothing like enough to hold a line against a determined force that concentrated at a chosen point. The same arithmetic that made the whole army too small for a 10,000 km perimeter applies locally.
What the physical remains suggest instead is control of movement. The wall has gates at every milecastle, roughly every 1.5 km, which is a strange feature in a defensive barrier and an obvious one in a customs post. It has a ditch and a road behind it, useful for moving men sideways along the frontier rather than for facing outward. And it is visible from a long way off, which is a message as much as an obstacle. Read it as a boundary that regulated and taxed traffic, marked where Roman jurisdiction began, and told everyone on both sides where they stood.
Now you. The Antonine Wall was built 160 km further north in the 140s, held for about twenty years, and then abandoned in favour of the older line. What does that sequence show?
Answer
That the frontier was a political object as much as a military one, and that it could be moved without much cost being paid either way. Antoninus Pius came to the throne in 138 with no military reputation and, like Claudius, needed a victory. Advancing in Britain provided one cheaply, and the new wall was turf rather than stone, which suits a line built for a reason that was not expected to outlive the reign. Withdrawal followed, and nothing catastrophic happened when it did, which is the informative part: if the wall had been the thing holding the province together, giving up 160 km of depth would have been a disaster. It was not. The frontier's exact position mattered far less than the existence of a frontier, and its position could accordingly be adjusted for reasons that had more to do with Rome than with Scotland.
Living without windfalls
A state that has stopped conquering loses three things at once, and it is worth being precise about them.
It loses the irregular income. Indemnities, plunder and confiscated royal treasuries had funded the Republic's wars and much of Augustus's building, and after Dacia there is no more of it. From then on, everything must come from taxation, which the earlier lesson showed was running at about five per cent of national income with no obvious way to increase it quickly.
It loses the main supply of slaves. Mass enslavement of defeated populations had stocked Italian agriculture, and with the wars over, the supply had to come from breeding, from the frontier trade and from exposure of infants, which is one reason slave prices in Egypt look higher in the second century than the first, and one reason tenancy gradually displaced slavery on Italian estates.
And it loses the standard proof that an emperor deserved the job. Roman political culture had measured a leader by victory for four hundred years, and an emperor who could not win a war of conquest had to manufacture the appearance of one, or find a different basis. Hadrian, who abandoned territory and built walls, was disliked in the Senate for exactly this reason, and the coins and inscriptions of the second century work hard to present the emperor as the provider of peace, security and prosperity rather than as a conqueror.
That last shift is the one that matters most for the century that follows. If the empire is no longer paying for itself with conquest, then it has to be worth having for what it produces in peacetime. The next lesson asks whether it was, and how anyone can possibly know, by counting broken pots, sunken ships and lead in the ice of Greenland.