The army that made them

The Roman army of the first century was the largest organised body of men in the western world, and it was not primarily a fighting force: it was a permanent, salaried, geographically fixed institution whose existence shaped the empire's taxes, its politics and its provincial society.

The previous lesson showed the legions choosing emperors in 69 and 193. That makes the composition of the army a political question rather than a military one, so this lesson asks what it actually was. Four things are worth carrying away: how many men there were, on what terms they served, where they stood, and what happened to a province that had them in it.

The order of battle

Augustus finished the civil wars with about sixty legions and reduced them to twenty-eight, which is the number the earlier course on the Republic ends with. Three of those were destroyed in the Teutoburg forest in AD 9 and never re-raised, leaving twenty-five, a figure that stayed close to constant for two centuries: Trajan had thirty, Septimius Severus thirty-three.

A legion was nominally about 5,000 men, all Roman citizens, organised in ten cohorts, with its own artillery, surveyors, engineers, medical staff and clerks. Beside it stood the auxilia, units of non-citizens raised in the provinces, in cohorts of infantry and alae of cavalry of roughly 500 men each, and by the early second century they were about as numerous as the legions. Almost all of the empire's cavalry was auxiliary. Add the nine praetorian cohorts, the urban cohorts and the fleets at Misenum and Ravenna, and the total under Tiberius is somewhere around a quarter of a million men under arms.

Example. Set 250,000 soldiers against the empire's population and its frontier, and say what kind of army that makes it.

Against a population of about sixty million it is one soldier per 240 inhabitants, which is a small army for a state that size. Against the frontier it is smaller still. The land and river frontier from the mouth of the Rhine round to the Sahara runs on the order of 10,000 km, so an even spread gives 25 men per kilometre, and the spread was not even. That arithmetic rules out one popular picture immediately: the frontier cannot have been a defended wall, because there were nowhere near enough soldiers to hold a line of that length against a serious attack. What the army could do was garrison the crossing points, patrol, escort, collect customs, and concentrate: pull ten thousand men from three provinces into one field force and destroy whatever had come across. It was a mobile striking force with a customs service attached, which is a different thing from a defensive perimeter, and the difference matters for what happens in the fourth century.

Now you. The same arithmetic gives one soldier per 240 civilians. What does that ratio tell you about how the provinces were held?

The terms

A legionary enlisted for twenty years under Augustus, in practice often longer, with four more years under the standards as a reserve; by the second century twenty-five years was normal. He swore an oath, the sacramentum, renewed each year, and it was sworn to the emperor by name rather than to the state. He could not legally marry while serving, a ban that lasted until Septimius Severus lifted it in the 190s, and which did not stop soldiers having families in the settlements outside the fortress, merely stopped those families having legal standing.

His pay under Augustus was 900 sesterces a year, delivered in three instalments, from which the army deducted food, boots and straps, hay for the unit's animals, clothing and a contribution to the camp's festival, with the remainder credited to a savings account held with the standards. Pay records on papyrus from Egypt show these deductions itemised line by line, which is how we know that the nominal figure and the money a soldier saw were different numbers. Domitian raised the pay to 1,200 sesterces, a rise of a third, and it stayed there for a century.

Example. In AD 14, on the news of Augustus's death, the legions in Pannonia mutinied and demanded a denarius a day, discharge after sixteen years, and no further service under the standards afterwards. Test the demand against the numbers.

A denarius a day is 365 denarii a year against the 225 they were getting, an increase of 62 per cent. Their actual daily rate was about 0.62 of a denarius, so they were asking for a round figure that happened to be a large rise, which is a familiar shape for a wage demand. The second and third clauses are the revealing ones. Discharge at sixteen years instead of twenty, and no reserve service after it, are demands about time rather than money, and they say that the grievance was less about the rate than about men in their forties still being held. Tacitus, who reports all this, is hostile to mutineers and gives their case anyway, in detail, including a centurion nicknamed Bring Another for the rate at which he broke his vine staff across soldiers' backs. When a hostile source preserves the other side's argument in usable form, that argument is usually authentic, because inventing a good case for your enemy is not what propaganda does.

Now you. The mutiny was settled and then, quietly, the concessions were withdrawn. Why could the state not simply have paid?

The citizenship machine

The auxiliaries were the part of the army that changed the empire most, and they did it by leaving it.

An auxiliary served twenty-five years as a non-citizen and received Roman citizenship on discharge, for himself and, in the earlier practice, his children, together with the right of legal marriage. The grant was recorded on a pair of small bronze tablets, wired and sealed with the names of seven witnesses, copying a master list posted in Rome. These are the military diplomas, and over a thousand of them survive, because a bronze certificate of your legal status is the sort of thing a family keeps and buries with you.

Consider the flow. If about 125,000 auxiliaries serve twenty-five year terms, roughly 5,000 men reach discharge each year, and each of them turns into a citizen household somewhere in a province. Over two centuries that is on the order of a million new citizen families, mostly in frontier regions, created by the army rather than by any political decision. That is the largest single engine of Romanisation in the empire, it ran automatically, and no emperor had to argue for it in the Senate.

Where the soldiers stood

The map is the politics. Under the early empire the great concentrations were on the Rhine, on the Danube, and in Syria facing the Euphrates. Britain held three legions after the conquest, Spain one, Africa one, Egypt two and later one. Italy had none: only the praetorians, the urban cohorts and the fleets.

That last fact is the constitutional heart of the system. The emperor governed, through his own appointed legates, exactly those provinces where the legions were, while the Senate appointed the governors of the peaceful ones. An Italian senator could reach the consulship and never in his life command an armed force. Power was therefore geographically fixed, and the fixing tells you where trouble comes from.

Example. Given the distribution, predict which provinces will produce challengers, then check the prediction against the two civil wars this course has already met.

Any command holding three or more legions, so the two German armies, the Danube commands in Pannonia and Moesia, Syria, and Britain. In 69 the challengers were Galba in Spain, backed by Gaul, Vitellius on the Rhine, and Vespasian with the Judaean and then the Danubian armies. In 193 there were exactly three: Septimius Severus in Pannonia, Pescennius Niger in Syria, Clodius Albinus in Britain. Six of the seven come from the multi-legion commands, and the exception, Galba, had to be adopted by the Gallic and German armies before he was anything. The prediction holds, and emperors could do the arithmetic as easily as we can, which is why Domitian forbade two legions to share a fortress and why later emperors kept splitting the large provinces into smaller ones. The whole administrative geography of the empire is partly a device for making sure no single governor commands enough men to try.

Now you. Egypt held two legions and a prefect of equestrian rank rather than a senator. Why the special treatment?

The army as a settlement

A legionary fortress was a town. Outside its walls grew the canabae, a civilian settlement of traders, craftsmen, innkeepers and the unofficial families of serving soldiers, and many of these became real cities: Bonn, Mainz, Vienna, Budapest, Belgrade, Leon, York and Chester all began as places where a legion stood.

The army also built. Roads, bridges, aqueducts, drainage and public buildings were made by soldiers with their unit's stamp on the bricks, which is how an archaeologist can date a wall to a legion and a decade. And it spread Latin: recruits from a dozen provinces served together, were commanded, paid and disciplined in Latin, and left after twenty-five years as Latin-speaking citizens.

Recruitment itself changed under the same pressure. Italians largely vanished from the legions during the first century; by the second, units recruited locally, so the army of the Rhine was largely raised on the Rhine and the army of Syria in Syria. That made the soldiers cheaper to find and rooted them in the region they defended, and it also meant that by the third century the men who chose the emperors were provincials, commanded increasingly by provincial officers, and the political weight of Italy fell accordingly.

What it cost, and what comes next

An institution of that size on fixed money wages is an accounting problem before it is anything else. A rough figure gives the shape: 125,000 legionaries at 900 sesterces and a similar number of auxiliaries on lower rates comes to something near 190 million sesterces of basic pay, before discharge bounties of some 84 million, before the praetorians at three times legionary rates, and before food, animals, weapons and building.

That is the largest recurring expense any pre-industrial state had ever taken on, and it fell due every year whether or not there was a war. The next lesson works out where the money came from: what the empire taxed, how much it raised, and how heavy the burden actually was on the people who paid it.