The Scordisci tribe settled in Syrmia in 279 BC after being forced to withdraw from their earlier campaigns. These people were part of a larger Celtic migration wave that disrupted the ancient Amber Road flowing from the Baltic Sea to Illyrian territories.
Tiberius defeated the Scordisci in 15 BC forcing them into an alliance following previous incursions into Roman territory. This military action occurred before the massive rebellion erupted in 14 AD involving Breuci and Daesitiates insurgents.
Pannonia Superior contained three legions while Pannonia Inferior held only one legion and one municipium as a border zone. Trajan divided Pannonia into these two regions between 103 and 107 AD to better manage different tribes along the frontier.
Rome completely handed over territory to Attila in 433 AD for subjugating Burgundians attacking Gaul. Flavius Aetius ceded parts of Pannonia to the Huns during the Migration Period in the fifth century after this transfer.
Diocletian split the country fourfold creating Pannonia Prima, Valeria, Savia, and Secunda with capitals at Savaria, Sopianae, Siscia, and Sirmium respectively. Each new province had its own capital established during this administrative restructuring.