The Roman Empire in 15 BC: How Augustus Expanded Rome

In 15 BC the Roman Empire under Augustus stretched from Spain to Syria and Egypt, and that summer Rome launched one of its most important campaigns of the era: the conquest of the Alps by Augustus’ stepsons Drusus and Tiberius. The campaign secured the mountain passes between Italy and the north, brought Raetia and the neighbouring Alpine regions under Roman control, and set the stage for Rome’s push into Germany. This article explains what the empire looked like in 15 BC, how Augustus expanded it, and how that expansion led to the disaster in the Teutoburg Forest in AD 9.

The Roman world in 15 BC

By 15 BC Augustus had been sole master of the Roman world for about fifteen years. He had defeated Mark Antony and Cleopatra at Actium in 31 BC, annexed Egypt in 30 BC, and in 27 BC received the name Augustus in a settlement that, in theory, restored the Republic while leaving him in control of most of the army.

The empire at this point included:

• Italy, the heartland, governed directly from Rome.
• Spain (Hispania), where the long Cantabrian Wars in the north-west had finally ended in 19 BC.
• Gaul, conquered by Julius Caesar in the 50s BC and now being organised into provinces.
• North Africa, including the old territory of Carthage.
• Greece, Macedonia and the Balkans as far as parts of the Danube region.
• Asia Minor, including the province of Galatia, added in 25 BC.
• Syria and the eastern frontier, where Augustus had recovered by diplomacy in 20 BC the standards lost to Parthia at Carrhae.
• Egypt, held as a special imperial possession governed by an equestrian prefect.

Yet there was an awkward gap at the centre of this map. The Alps, directly north of Italy, were still home to independent peoples who controlled the passes and could raid the valleys of northern Italy. Rome’s armies moving between Italy, Gaul and the Balkans had to go around or negotiate. Closing that gap was a strategic priority.

Why the Alps mattered

Controlling the Alpine passes would do three things. It would protect northern Italy from raids. It would give Rome direct routes from Italy to the upper Rhine and the upper Danube. And it would link Gaul in the west with Illyricum in the east, turning a scattered set of conquests into a more continuous territory.

There was also a more immediate trigger. In 16 BC, a Roman army under the governor Marcus Lollius suffered a humiliating defeat on the Rhine at the hands of Germanic raiders led by the Sugambri, losing a legionary eagle. Augustus travelled to Gaul himself and remained there for several years, reorganising the provinces and planning the next steps. The Alpine campaign came the following year.

The Alpine campaign of 15 BC

The campaign was a pincer movement led by Augustus’ two stepsons, the sons of his wife Livia from her first marriage.

Drusus from the south

Nero Claudius Drusus, the younger brother, advanced north from Italy into the mountains. Ancient accounts describe him defeating Alpine peoples in the valleys of the Adige region and pushing through toward the upper Danube. Drusus had already fought Alpine peoples in the previous year.

Tiberius from the west

Tiberius, the future emperor, advanced from Gaul. Ancient writers report that he used boats on Lake Constance (Lacus Brigantinus) and moved into the lands of the Vindelici, north of the Alps toward the Danube.

The result

The two armies converged, and within a single campaigning season the core of the region was subdued. The conquered territory became known as Raetia and Vindelicia, later organised into the province of Raetia, covering parts of modern Switzerland, southern Germany, Austria and northern Italy. The source of the Danube was reached by Roman forces in this period.

Augustus did not command in person, but the victory was celebrated as his. The poet Horace wrote odes praising the achievements of Drusus and Tiberius, part of a wider effort to present the imperial family as the source of Rome’s success.

Noricum

East of Raetia lay Noricum, a Celtic kingdom in the region of modern Austria, famous for its iron. Noricum had long-standing trade ties with Rome and appears to have come under Roman control around the same time, largely without major fighting. It was later organised as a province.

The Tropaeum Alpium: Augustus’ monument in the mountains

To commemorate the conquest of the Alps, the Senate dedicated a huge monument at La Turbie, above modern Monaco, around 7/6 BC. Known as the Tropaeum Alpium or Trophy of Augustus, it stood at the highest point of the Roman road along the coast between Italy and Gaul.

The monument carried an inscription honouring Augustus and listing the Alpine peoples brought under Roman rule. The Roman writer Pliny the Elder copied the text, which names dozens of tribes, and fragments of the inscription were later recovered at the site. The trophy was partly dismantled over the centuries but was restored in the 20th century and can still be seen today. It is one of the clearest physical statements of how Augustus wanted his conquests remembered.

Settling Gaul and Spain

While the Alps were being conquered, Augustus was reorganising the territories Rome already held.

Gaul was divided into provinces: Narbonensis in the south, a long-settled area, and the “Three Gauls” of Aquitania, Lugdunensis and Belgica. Augustus ordered census surveys for taxation, founded and developed cities, and built roads. In 12 BC Drusus dedicated an altar to Rome and Augustus at Lugdunum (modern Lyon), where representatives of the Gallic communities gathered each year. This created a focus of loyalty that bound local elites to the imperial system.

Spain had taken nearly two centuries to conquer. Augustus campaigned there himself in the 20s BC, and his general Agrippa ended resistance in the north-west in 19 BC. Augustus then reorganised the peninsula into three provinces: Baetica in the south, Lusitania in the west and Tarraconensis covering the rest. Veteran colonies such as Emerita Augusta (modern Mérida) and Caesaraugusta (modern Zaragoza) followed.

Augustus’ reforms: the machinery behind expansion

Senatorial and imperial provinces

In the settlement of 27 BC, the provinces were divided. Peaceful, long-established provinces such as Africa and Asia were governed by proconsuls chosen by the Senate. Frontier and newly conquered provinces, where most of the legions were stationed, were placed under Augustus, who governed them through legates he appointed. The arrangement looked like power-sharing but in practice gave Augustus control of the army.

A standing professional army

Under the Republic, armies were raised for particular wars and loyalty often went to individual generals. Augustus created a permanent professional army with fixed terms of service and set rewards on discharge. Legions were given permanent numbers and stationed in the frontier provinces. Alongside the citizen legions, non-citizen auxiliary units provided cavalry, archers and light infantry. In Rome itself, the Praetorian Guard protected the emperor.

In AD 6 Augustus established a military treasury (aerarium militare), funded by new taxes, to pay discharge bonuses to veterans. Tying soldiers’ futures to the state, rather than to individual commanders, was one of his most important reforms, designed to prevent the civil wars that had destroyed the Republic.

Roads and communication

New and improved roads, including routes over the Alps built after the conquest, connected the provinces. Augustus also set up a courier system to carry official messages across the empire.

From the Alps to Germany

With the Alps secured and Gaul organised, Augustus turned to the lands beyond the Rhine. Historians still debate his goal. Some believe he aimed to push the frontier to the Elbe River and create a province of Germania; others think he was mainly trying to secure Gaul by intimidating Germanic peoples. Either way, the result was nearly two decades of campaigning.

Drusus’ campaigns (12 to 9 BC)

Drusus led a series of campaigns across the Rhine, using both land forces and a fleet on the North Sea coast. In 9 BC he reached the Elbe. On the return march he was injured in a fall from his horse and died. His brother Tiberius reportedly rode a great distance to reach him before his death, and then escorted his body back to Rome.

Tiberius and consolidation

Tiberius took over operations in Germany and campaigned there in the following years. After a period away from public life, he returned to Germany in AD 4 to 5. Roman bases were established east of the Rhine, and Roman-style settlement began, as the excavated town at Waldgirmes in Hesse has shown.

A planned attack on the Marcomanni in Bohemia was abandoned in AD 6 when a massive revolt broke out in Pannonia and Illyricum. Suppressing it took three years and much of Rome’s military strength.

The Teutoburg Forest, AD 9

In AD 9 the governor in Germany, Publius Quinctilius Varus, was leading three legions, the 17th, 18th and 19th, along with auxiliaries, when he was ambushed. The attack was organised by Arminius, a chieftain of the Cherusci who had served with the Roman army and was trusted by Varus. Over several days the Roman column was destroyed in difficult wooded country. Varus took his own life.

The site has long been debated, but since the late 1980s excavations at Kalkriese, near Osnabrück in Lower Saxony, have uncovered extensive Roman military equipment and coins linked to the battle. Many, though not all, scholars identify Kalkriese as at least one part of the battlefield.

The ancient biographer Suetonius records that Augustus was deeply shaken by the news and would cry out for Varus to give him back his legions. The three legion numbers were never used again. Rome launched punitive campaigns under Germanicus in the following years, but it did not attempt to hold a province east of the Rhine in the same way again. The Rhine and Danube became the long-term frontier.

Why 15 BC matters

The Alpine campaign of 15 BC is often overshadowed by Actium before it and Teutoburg after it, but it was a turning point. It completed Rome’s control of the arc of mountains around Italy, connected the western and eastern parts of the empire by land, and created the base from which Rome pushed to the Danube and into Germany. The limits that emerged after AD 9, the Rhine and the Danube, would shape European history for centuries.

Augustus left his successors advice, reported by Tacitus, to keep the empire within its existing boundaries. Whether that reflected strategic wisdom or the shock of Teutoburg, it marked the end of the great age of Augustan expansion.

Frequently asked questions

Who ruled Rome in 15 BC?

Augustus, the first Roman emperor, ruled Rome in 15 BC. He held power from his victory at Actium in 31 BC until his death in AD 14.

What did Rome conquer in 15 BC?

In 15 BC Augustus’ stepsons Drusus and Tiberius conquered the central Alpine region, including the lands of the Raeti and Vindelici, later organised as the province of Raetia. Noricum, to the east, came under Roman control around the same time.

What is the Tropaeum Alpium?

It is a monument at La Turbie, near Monaco, dedicated around 7/6 BC to celebrate Augustus’ conquest of the Alpine peoples. Its inscription listed the conquered tribes and was recorded by Pliny the Elder.

How large was the Roman army under Augustus?

After the civil wars Augustus reduced the number of legions and settled on a permanent professional army of about 28 legions until the loss of three at Teutoburg in AD 9, supported by a similar number of auxiliary troops. Exact totals are estimates.

Why didn’t Rome conquer Germany?

Rome campaigned extensively east of the Rhine, but the destruction of three legions in the Teutoburg Forest in AD 9, combined with the strain of the Pannonian revolt, led Augustus and his successors to treat the Rhine and Danube as the frontier.

Further reading

Augustus: First Emperor of Rome by Adrian Goldsworthy. A detailed biography covering Augustus’ reforms, his family and his military campaigns. Find it on Amazon

The Battle That Stopped Rome by Peter S. Wells. An archaeologist’s account of the Teutoburg Forest disaster and the evidence from Kalkriese. Find it on Amazon

SPQR: A History of Ancient Rome by Mary Beard. Places Augustus’ rise and settlement in the long story of Roman power. Find it on Amazon

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