Julius Caesar invoked the threat of Germanic attacks as one justification for his annexation of Gaul ( modern France ) to Rome.

As Rome expanded to the Rhine and Danube rivers, it incorporated many Celtic societies into the Empire.

The Germanic tribal homelands to the north and east emerged collectively in the records as Germania. The peoples of the Germania were sometimes at war with Rome, but also engaged in complex and long-term Alps relations, military alliances, and cultural exchanges with Rome as well.

The initial purpose of the Roman campaigns was to protect Trans-Alpine Gaul by controlling the area between the Rhine and the Elbe.

In 58 B.C. Julius Caesar, governor of the Roman province of Southern Gaul, conquered the remainder of Gaul, which had been free until then: Thus, for the first time, the powerful Roman Empire moved into immediate vicinity to Germania, and further expansion and colonization on part of the Germanic tribes were blocked.

Caesar defeated Germanic warlord Ariovist, who had tried to conquer Gaul himself, and pushed back the Germanic Tencterians, who had crossed the Rhine from Upper Hesse. He had a 400-meter bridge built over the Rhine in 10 days, marched to the Germanic right bank of the Rhine, showed off the power of his army, won over the Germanic Ubians as allies and forced some other tribes into peace agreements.

In 38 B.C. Augustus' general Agrippa resettled the Germanic Ubians, allied with Rome, in a new town at the left bank of the Rhine in order to protect Roman Gaul from raids by uncontrolled Germania. This was the founding of Colonia Agrippinensis, today's City of Cologne.

The wealthy country of Gaul seemed firmly and safely in the hands of the Romans. Even before the Roman conquest, the Gauls had already lived in towns, and they started to get used to living under Roman rule. But in 16 B.C. Gaul was raided by the Germanic Sugambrians, Usipians, and Tencterians. They severely defeated Roman governor Lollius and freely looted the wealthy country and then returned to their homeland with heavy booty.

Emperor Augustus had led many wars, but this was the heaviest defeat his forces had suffered so far. Though Gaul was only looted, these attacks made Rome afraid that one day it could lose Gaul, a country that by then was yielding more taxes and crop than the fabulously wealthy Egypt.

In order to avoid this danger in the long run, Germania had to be conquered - though the country itself neither offered cities, nor treasures, nor a food surplus.

Augustus moved to the Rhine border and prepared the big offensive in person. First, all the territory between the Alps and the Danube was to be conquered, and then Germania was to be attacked simultaneously from the Rhine, the Danube, and from the North Sea coast with a fleet.

As a starting point, the Romans established 50 legion camps along the Rhine and connected them by army routes (these camps turned into modern cities Xanten, Bonn and Mainz).

Along the left bank of the Rhine, a considerable Roman fleet was being built. Emperor Augustus appointed his adoptive son Drusus governor of Gaul and made him commander-in-chief of the Rhine troops - probably 5 to 6 legions, or about 50,000 men, expected to conquer Germania.

While the Romans were preparing the war against Germania from their province of Gaul, the Gauls were embittered over the Roman tax collection: Apparently several Gaullic tribes were ready to risk an uprising against the Roman rule. But Drusus fell with his horse, broke his thigh, and died of wound-fever after one month, being only 29 years old.
He had been successful, and popular with Romans, and favoured by the Emperor: It is likely he would have become Augustus' successor - instead of his uncanny brother Tiberius.

After the death of the victorious general Drusus, 33 years-old Tiberius assumed continuation of the war. In the spring of 8 B.C., he once again crossed the Rhine with a large army.

The Germanic tribes were too weakened from the continuous warfare of the last years to put up any resistance: For four years, they had been attacked every year by superior Roman armies, their settlements had been regularly burnt down, and their fields devastated. In the countless bloody battles and skirmishes during these four years, probably all tribes had lost a significant proportion of their men.

Already in the previous year, the allied tribes had been unable to prevent Drusus' army from marching through their territories. This year promised to be equally unsuccessful. It seemed better to capitulate now - and not to wait until one would be totally defeated and defenseless. Probably out of these considerations, all Germanic tribes sent envoys to the Romans, asking for peace.

In an unheroic way that was typical of him, Emperor Augustus simply had all these men arrested, and had them brought to several Roman cities as hostages and they evaded this imprisonment by resorting to suicide. Now the Romans concluded peace treaties with all Germanic tribes: All tribes recognized the Roman rule, and started to pay tribute and provide troops for the Romans.