Illyricum became a formal administrative unit in 27 BC when Augustus established the province. Initially it was a senatorial propraetorial province before becoming imperial in 11 BC under Publius Cornelius Dolabella.
Lack of water and poor or arid soil made much of Illyria a poor agricultural area which gave rise to piracy among local communities who relied on the sea rather than farming. The Dinaric Alps created a barrier between the coastal plain and the interior while numerous islands off the coast lacked drinking water.
Two leaders named Bato commanded rebel forces during the four-year conflict: one from the Daesitiate tribe and another from the Breuci. Tiberius conducted counterinsurgency operations against them until all of Pannonia eventually laid down arms at the River Bathinus.
Emperors including Claudius II Gothicus, Quintillus, Aurelian, Probus, Diocletian, Constantine the Great, and Justinian I emerged from cities such as Sirmium, Salona, Naissus, and Tauresium. These rulers reigned from the third century through the sixth century with some like Justinian serving from 527 to 565 AD.
Western Illyricum formed the Diocese of Pannonia covering modern western Hungary almost all Croatia and Bosnia Herzegovina and northern Serbia. Eastern Illyricum became part of the Praetorian Prefecture including central southern Serbia Montenegro Albania North Macedonia Greece and western Bulgaria.