Province of the Roman Empire, corresponding to present western Hungary and parts of eastern Austria, Slovenia, and northern Yugoslavia (Vojvodina) incorporated into the Roman Empire in 10 AD. (Roman conquest began there about 35 BC under Octavian.) Pannonia was a prosperous Roman frontier province with numerous villas. The principal town was Aquincum (part of modern Budapest). The emperor Trajan divided the province about 106 AD. The western and northern districts constituted Pannonia Superior, which was the focal point of the Roman wars with the Marcomanni during the reign of Marcus Aurelius (161-180). The southern and eastern districts were organized as Pannonia Inferior under Diocletian (284-305). Pannonia Superior was divided into Pannonia Prima and Pannonia Ripariensis (or Savia), and Pannonia Inferior was divided into Valeria and Pannonia Secunda. Pannonia was the birthplace of several Roman emperors of the 3rd century, and the province provided large numbers of troops for the Roman army. The grave barbarian threat in the 4th century AD forced the Romans to withdraw after 395. The Pannonians were mainly Illyrians, but there were some Celts in the western part of the province.