Illyria was a country that, in ancient times, stretched along the eastern side of the Adriatic Sea, from Epirus northward. Illyria (Latin Illyricum) was not a homogeneous territory, but varied in extent at different periods in history. The region was inhabited by numerous tribes, who seem seldom to have been held together by any sort of political cohesion. From some cause or other – probably the mountainous character of the region they inhabited was the principal cause – they were the last of the peoples of the Balkan peninsula to be brought within the fold of civilization. The Greek colony of Dyrrhachium or Epidamnus, in the south, was the only point of access for the surrounding Greek culture.
The Illyrians are described as resembling the savage Thracians in their manners, as tattooing their bodies, as offering human sacrifices to their deities, but as honoring women, who even held chieftainships among them. For many years they seem to have kept up a series of incessant attacks upon the early kings of Macedonia; they levied tribute from Amyntas II, and slew Perdiccas (359 BC). But they were subdued by Philip II and Alexander, who annexed their country to Macedonia.
In the third century, after the breaking up of the Macedonia monarchy, they aggravated Greece and Italy by their piratical excursions. Eventually the patience of Rome was exhausted, and in two short wars (229 and 219 BC) the Romans succeeded in subjugating the Illyrians. Fifty years later they provoked a third war with Rome, which resulted in their defeat and the incorporation of their territories in the republic. Nevertheless, the Illyrians only consented to be civilized at sword's point and they frequently revolted against their conquerors; but in 35 BC Illyria was made a Roman province. During the empire they served faithfully in the Roman armies, and even gave half-a-dozen emperors to the state, as Claudius II, Aurelian, Diocletian, Probus, and some others. Under the rule of the emperors the political importance of Illyria, or Illyricum as the Romans called it, greatly increased. In the second century BC. Illyria extended as far north as the Danube, and even beyond it, and included Noricum, Pannonia, Moesia, Thrace, and Dacia. Constantine further enlarged its boundaries, and made it one of the four chief divisions of his empire. But when the empire was divided between East and West, Illyria was also divided. Noricum, Pannonia, Moesia, etc, were designated as Illyria Barbara, and incorporated within the empire of the West; Illyria Graeca, embracing Greece, Macedonia, Epiris, etc, was attached to the eastern empire. In the period of the final dissolution of the western empire Illyria was successively overrun by the Goths, the Huns, and several Slavic tribes, and nearly all traces of civilization disappeared. The Illyrians themselves partly amalgamated with the Huns and their Slavic conquerors, and partly were driven southward, where one of their tribes, the Albani, survive, at all events in name, in the modern Albanians. As the several Slavic states became consolidated and rose to power, the political importance of Illyria, and even its name, gradually died away.
The name was revived when Napoleon, in 1809, formed the territories he had wrested from Austria into the Illyrian provinces. In 1816, when they were restored to Austria, this power constituted out of them and the provinces of Carinthia, Carniola, Görz, Gradisca, and Istria the kingdom of Illyria. But the designation was dropped in 849, and the territories included in it were reorganized as provinces.