peter clings
Pannonia just before the arrival of the Magyar tribes
The importance of a Roman legacy of roads, and ruins in the formation of the early medieval states in Europe can be reflected also in the early arrival of feudalism in the South Eastern Europe. The Bulgarian Czardom had its roots back in the 7th century, post Roman provinces of the lower Danube
Specific to former provinces of Pannonia though, I believe that there started to flourish the Great Moravia, a border Duchy of the Carolingian Empire. It was located there for the excellent infrastructure from the Roman times such as roads, ruined forts and churches that could be repaired, etc
On the map below, I am highlighting the region where Great Moravia had flourished before the arrival of the Tourkoi (I am using the Byzantine terminology for the Magyars or proto Hungarians) and an arrow pointing to the old corridor that was left under developed by the Romans in antiquity for the friendly Barbarian

Upon their arrival, the Tourkoi were not friendly at all. They had been pushed out by their co-linguistic enemy the Pecheneg people from the Eurasian plains as it was cold and to keep pasture for hoses required strong nomad fighting. The Tourkoi were literally hungry having been chased out of the steppe. No offence, but people started to take notice on that too. In a short succession of event, as any raider nomads before and after them, they started robbing around. All that came into their pathway, they destroyed. Including the Great Moravia. There is a very interesting chapter dedicated to that event in the De administrando imperio
The Tourkoi were into the robbing business throughout Europe, reaching as far as the Moorish Hispania for one generation of plunder and devastation. Eventually, they started to get some backlashes too from the local people. A major blow came from the German Kingdom under Otto the Great. The Battle of Lechfeld seems to had been terminal. Some of the remaining Tourkoi attempted to still attack the Byzantines but were made to behave sometimes by force, but mostly with gold and Byzantine made gadgets (smoke and mirror that is :-)). There is a very strange transition period between 910 AD to the crowning of Stephen I of Hungary that I don’t understand well. He received a major support from his German father in law, in knights, clergy and money so he was able to wipe out the local competition for power. Meantime the Tourkoi became settled and in linguistic terms they seem to have become something else, the Ungeroi. But they kept attacking the Byzantines all the time. From Stephen I, the newly established Medieval Kingdom of Hungary started to grow in economic terms and prestige
In a nutshell, the Pannonian basin comprised the developed section inhabited by the Slavonic people, and the non developed section east of Danube bordering the Carpathian mountains and forests in the East. The Hungarians found and fought an early Slavonic principality called Great Moravia, and to the east, they found some Romance speaking Shepherds, the western fringes of the population in the former Roman province of Dacia (later known by the name Transsilvania, or Transylvania), whom they did not fight but only trade with. To the South, they encountered the Bulgarians with who they crossed arrows a few times indecisively though