Constantinople and the East, led across Illyricum from Dyrrachium to Thessalonica. In the 5th century began a series of invasions which profoundly modified the ethnical character and the civilization of the Illyrians. In 441 and 447 their country was ravaged by the Huns. In 481 Dalmatia was added to the Ostrogothic kingdom, which already included the more northerly parts of Illyricum, i.e. Pannonia and Noricum. Dalmatia was partially reconquered by Justinian in 536, but after 565 it was devastated by the Avars, and throughout the century bands of Slavonic invaders had been gradually establishing themselves in Illyria, where, unlike the earlier barbarian conquerors, they formed permanent settlements. Between 600 and 650 the main body of the immigrants occupied Illyria (see SERVIA: _History_; and SLAVS). It consisted of Croats and Serbs, two groups of tribes who spoke a single language and were so closely related that the origin of the distinction between them is obscure. The Croats settled in the western half of Illyria, the Serbs in the eastern; thus the former came gradually under the influence of Italy and Roman Catholicism, the latter under the influence of Byzantium and the Greek Church. Hence the distinction between them became a marked difference of civilization and creed, which has always tended to keep the Illyrian Slavs politically disunited. The Croats and Serbs rapidly absorbed most of the Latinized Illyrians. But the wealthy and powerful city-states on the coast were strong enough to maintain their independence and their distinctively Italian character. Other Roman provincials took refuge in the mountains of the interior; these Mavrovlachi, as they were called (see DALMATIA: _Population_; and VLACHS), preserved their language and nationality for many centuries. The Illyrian tribes which had withstood the attraction of Roman civilization remained unconquered among the mountains of Albania and were never Slavonized. With these exceptions Illyria became entirely Serbo-Croatian in population, language and culture.