The Roman province of Illyricum replaced the formerly independent kingdom of Illyria. It stretched from the Drilon river in modern Albania to Istria (Croatia) in the west and to the Sava river (Croatia) in the north. Salonae (near modern Split in Croatia) functioned as its capital.
The Roman Republic finally conquered the kingdom of Illyria in 168 BC by defeating the army of the Illyrian king Gentius. From 167 BC, southern Illyria became a formally independent Roman protectorate. The region had considerable strategic and economic importance for the Romans. It possessed a number of important commercial ports along its coastline, and had gold-mines in its interior regions. Illyria also became the starting point of the Via Egnatia, the great Roman road that ran from Dyrrachium (modern Durazzo), on the Adriatic, to Byzantium in the east.
In 59 BC the Lex Vatinia assigned Illyricum (together with Cisalpine Gaul) as a provincia (zone of responsibility rather than a "province" as understood today) to Julius Caesar. The Roman administration did not establish a province until Octavian's wars in Illyricum in the period 35-33 BC: the first mention of the province of Illyricum occurs in the context of Augustan settlement of 27 BC.
As the Romans expanded their power in the region through a series of campaigns known as the Pannonian War or as the Pannonian wars (Bellum Pannonicum, 12-9 BC), fought against group of peoples known as the Pannonians, they enlarged the province of Illyricum. After crushing a revolt of Pannonians and (also Dalmatians) Daesitiates — known as the rebellion of Bato (Bellum Batonianum, 6-9 AD) — subsequent to 10 (some scholars such as Jeno Fitz move this date to middle-late Claudian era c. 20-35), Roman administrators dissolved the province of Illyricum and divided its lands between the new provinces of Pannonia in the north and Dalmatia in the south. But the name continued in use to refer to the region. The emperor Diocletian set up the Praetorian prefecture of Illyricum as one of four prefectures that he established; it encompassed Pannonia, Noricum, Crete, and the whole Balkan peninsula except Thrace.
The region's native peoples won renown for their military prowess and they became an important source of manpower for the Roman army. Several notable Roman emperors came from the region, including Aurelian, Claudius II, Constantine I and Diocletian, as well as the Byzantine emperors Anastasius I and Justinian I.