The Roman invasion of Boiohaemum (Latin for "the home of the Boii"; present Bohemia), planned by Tiberius, has already been launched from two directions when news comes in CE 6 that Pannonia and Illyricum have revolted, posing the gravest threat to Italy since Hannibal's invasion.

At risk is the strategic province of Illyricum, recently expanded to include the territory of the Pannonii, the indigenous communities inhabiting the region between the rivers Drava and Sava, who had been subjugated by Rome in 12-9 BCE.

Illyricum is on Italy's eastern flank, exposing the Roman heartland to the fear of a rebel invasion.

Augustus accordingly orders Tiberius to break off operations in Germany and move his main army to Illyricum.

Tiberius sends Marcus Valerius Messalla Messallinus (the governor of Dalmatia and Pannonia) ahead with troops.

Other natives are recruited to fight against the Marcomanni as the rebellion swiftly overtakes enormous areas of the western Balkans and the Danube region.