Illyria, a district of the Balkan Peninsula, which has varied in extent at different periods. To the Greek geographers Illyria (e Illuris or Illurikon) connoted the eastern shore of the Adriatic Sea and the adjoining mountainous territory stretching into the interior, all of which was the abode of Illyrian tribes. One section of the Illyrian people had migrated to Italy, first to central Italy, where there are traces of them in Picenum and Umbria; later, towards the middle of the eighth century B.C., the Japyges crossed to Apulia and Calabria; and, at the beginning of the seventh century B.C., the Veneti to northern Italy and what is now Carinthia. Even the Illyrians who remained behind never achieved national unity. The kingdom of Bardylis and his son Kleitos, who settled in Macedonia, rose to some importance in the fourth century B.C., until they were subdued by King Philip in 357 B.C. and Alexander the Great in 335 B.C. About 250 B.C. the tribes known as the Ardriaii and Antariates, under the princes Pleuratos and Agron, terrorized the sea with their fleets and preyed on the Greek colonies on the eastern coast of the Adriatic and the neighboring islands (Pharos, Corfu, etc.). Rome when called on by Issa, one of these Greek cities, took a hand in Illyrian affairs for the first time, and put an end to this peril. When Genthius, the Illyrian king, took sides with Perseus during the last stand of the Macedonians against Rome (171-168 B.C.), he was banished by the Romans, his kingdom left to disintegrate, and later converted into a Roman province (59 B.C.). Part of the remaining Illyrian tribes submitted voluntarily, and the rest were brought under the Roman yoke by Augustus (23 B.C.). From the time of Augustus the name Illyria was applied not only to the present Province of Illyria, since 11 B.C. a province of the empire and called Dalmatia (embracing the Dalmatia of today, Montenegro, the western part of Croatia, and the northern part of Albania), but was made to include the districts of Rhaetia, Noricum, Pannonia, Mcesia, and Macedonia. At the time of the division of provinces under Hadrian, it was subdivided into seventeen provinces, comprising also Thrace. When Constantine the Great in A.D. 324 divided the entire Roman Empire into four prefectures, Illyricum, as one prefecture, was assigned to Western Rome, the residence of the praetorian prefect being Sirmium. On the accession of Theodosius I (379), the prefecture was divided into Eastern and Western Illyricum, the former embracing the two civil dioceses of Macedonia, including Epirus, Thessaly, and Greece, and Dacia, under the jurisdiction of a praetorian prefect residing at Thessalonica (Saloniki). Western Illyricum was placed as a civil diocese under the authority of a vicar of the prefect of Italy residing at Sirmium. In 379, or more probably, not until 395, Eastern Illyricum became a part of the Eastern Empire (cf. Rauschen, “Jahrbucher der christlichen Kirche unter dem Kaiser Theodosius dem Grossen”, Freiburg, 1897, 469-73).
Ecclesiastically, the whole of Illyricum, which had first received Christianity from St. Paul the Apostle, and Titus, his disciple, was from the first under the Bishop of Rome, as the Patriarch of the West, and, after the division of the empire, formed the eastern part of the territory subject to the pope, as Patriarch of Rome, although politically a part of Byzantium. As the patriarchs of Constantinople endeavored to extend their patriarchal authority over Eastern Illyricum, the popes sought to preserve intact their jurisdiction over the eastern part of Illyria by appointing the bishops of Thessalonica papal vicars for Illyricum. The first of these vicars is said to have been Bishop Acholius or Ascholius, (d. 383 or 384), the friend of St. Basil. His successor, Anysius, was confirmed by Pope Damasus and his successor, Pope Siricius, as representative of the Roman See. In like manner, the succeeding popes, Anastasius I and Innocent I, extended the powers of the bishops of Thessalonica over Illyria. The authority vested in the bishops of Thessalonica over the metropolitans and other prelates of Illyria was substantially that usually enjoyed by a patriarch, except that patriarchal power is ordinary and attached to a definite see, while the jurisdiction of the vicars of Thessalonica was delegated; they exercised the patriarchal authority belonging to the pope, as his special commissary. The papal Vicariate of Thessalonica persisted for a century with practically no interruption until the connection was weakened by the first Greek schism, brought about by Acacius, Patriarch of Constantinople (471-89), and Petrus Mongus of Alexandria over the “Henoticon“. The bishops of Illyria withdrew from communion with Rome, without attaching themselves to Constantinople, and remained for a time independent. Not until Dorothea, Bishop of Thessalonica, declared for the intruded patriarch, Timotheus, did forty Illyrian bishops renounce allegiance to him (515) and proclaim to Pope Hormisdas their loyalty to Rome.
After the suppression of the Acacian Schism, the vicarship of the bishops of Thessalonica does not seem to have been immediately restored, owing to the policy of the Byzantine emperors, Zeno and Anastasius; still they enjoyed a certain precedence over the other Illyrian bishops. When, in 541, Justinian I, to increase the prestige of his native city Scupi (the present Skoplje or Uskup), raised the bishop of that city to the rank of Archbishop of Justiniana Prima, and placed him over the ecclesiastical provinces of the civil diocese of Dacia, the vicarship was restored without consulting Pope Agapetus, but was divided between the Metropolitan of Thessalonica, for the provinces in which Latin was spoken, and the Metropolitan of Justiniana Prima, for those in which Greek was the native tongue. Pope Vigilius (c. 545) was the first to give his approbation to this arrangement. The title of papal vicar was henceforth almost an honorary title, as the popes, in the exercise of their patriarchal power, now dealt, for the most part directly with the individual bishops. At first the political situation was in their favor, Italy and Illyricum being both under the Eastern Empire. But even after a large part of both lands had been lost to the Byzantine Empire, Illyricum remained entirely under the jurisdiction of the Western patriarchs, the popes, as for example Gregory the Great and Martin I, who exercised their metropolitan authority, with-out any objections on the part of the Eastern emperors or the patriarchs of Constantinople. As late as the middle of the eighth century, the ecclesiastical Provinces of Eastern and Western Illyricum were undoubtedly within the Patriarchate of Rome. Soon afterwards, however, they began gradually to withdraw from communion with Rome, and the patriarchs of Constantinople succeeded in bringing Illyria under their jurisdiction. Even Pope Nicholas I attempted in vain to recover the ancient privilege of the Roman See to appoint the Bishop of Thessalonica as his vicar. From the end of the ninth century Eastern Illyria appears in the “Notitiae episcopatuum” as wholly within the Patriarchate of Constantinople, with which it was involved in the Great Schism.
Meanwhile political changes of a far-reaching nature were taking place. Towards the end of the sixth century Western Illyria was overun by Avars and Slavic tribes, and at the beginning of the seventh century was occupied by Croats and Serbs. These gradually developed into the Slavic kingdoms of Dalmatia and Croatia, whose history was one of varied fortunes until at last they came under the authority of the Hapsburgs. Nothing but the eastern coast and the islands of the Adriatic remained under Byzantine control, and these only until the eleventh century, when the rising Republic of Venice began to establish her authority there. The Byzantine rule was of longer duration in Eastern Illyria, but even there was frequently threatened and weakened by Serbs and Bulgars, until in the fourteenth and fifteenth centuries the Osmans conquered the whole Balkan Peninsula. The name of Illyria then disappeared from history, only to acquire new significance through the modern history of Austria. Under Leopold I (1656-1705) the Serbs or Raizi, who had been established on Hungarian territory since 1690, were designated as the Illyrian nation; to provide for their protection against Magyar incursions a special office was created at the Court of Vienna, known as the Illyrian Court Deputation, which was abolished in 1777, and in 1791 enjoyed a brief revival as the “Illyrian Imperial Chancery”. Napoleon united the territories on the Adriatic Sea, ceded by Austria in the Peace of Schonbrunn, in 1809, with Croatia and Ragusa, under the title of the “Seven Illyrian Provinces”, made them a part of the French Empire, and placed their administration in the hands of a governor general (Marmont, Funot, and Fouque). After his fall the territories reverted to Austria, and were constituted, together with the islands, a kingdom of Illyria (1816), with two seats of government. In 1822 the civil district of Croatia and the littoral were separated and united with Hungary; the organization of the year 1849 did away entirely with the Kingdom of Illyria, resolving it into the crownlands of Carinthia, Carniola, and the coast lands (Gorz and Gradiska; Istria; and Triest).