Conflict between the Illyrians and Romans started in 229 BCE, but Rome wouldn't complete its annexation of the region until 9 CE. In the Roman period, Latin-speaking settlers from all over the Roman empire settled among the Illyrians and Roman soldiers were encouraged to retire in the region.Christianity had already arrived in the region by the end of the 1st century, and numerous artifacts and objects from the time testify to this. Following events from the years 337 and 395 when the Empire split, Dalmatia and Pannonia were included in the Western Roman Empire. The region was conquered by the Ostrogoths in 455, and further exchanged hands between the Alans and Huns in the years to follow.
By the 6th century, Emperor Justinian had re-conquered the area for the Byzantine Empire. The Slavs, a migratory people from southeastern Europe, were allied by the Eurasian Avars in the 6th century, and together they invaded the Eastern Roman Empire in the 6th and 7th centuries, settling in what is now Bosnia and Herzegovina and the surrounding lands. Heraclius to drive the Avars from Dalmatia