Early Slavs settled in the eastern and southern parts of the former Roman province of Pannonia. The term Lower Pannonia (, , , ) was used to designate those areas of the Pannonian plain that lie to the east and south of the river Rába, with the division into Upper and Lower inherited from the Roman terminology.
From the middle of the 6th to the end of the 8th century, the region was under the domination of the Avars, while the Slavic inhabitants lived under Avar rule. By the beginning of the 9th century, that state was destroyed and replaced by the supreme rule of the Frankish Empire, which lasted until the Magyar conquest (c. 900).
During the Frankish period, the region of Lower Pannonia was governed by local Slavic rulers, who were under the suzerainty of Frankish kings. Within the Frankish administrative system, the March of Pannonia was created, with direct Frankish rule exercised in Upper Pannonia through Frankish counts, while Lower Pannonia was governed as a principality by local Slavic princes, under the supreme Frankish rule. During the 9th century, Frankish domination in Lower Pannonia was also contested by the Bulgarian Empire and Great Moravia.
By the 10th century, the Hungarian conquest of the Carpathian Basin effectively split the Slavic communities in the region in two, leading to the formation of the West Slavs and the South Slavs.
Background
Roman rule in Pannonian regions collapsed during the 5th century, and was replaced by subsequent domination of Huns, Goths and Langobards. During the reign of Byzantine Emperor Justin II (565–578), and following the Lombard-Gepid War in 567, Pannonia was invaded by Avars who subsequently conquered almost entire Pannonian Plain (568). Although it is possible that some small Proto-Slavic groups could have arrived in the "middle of the 5th century and the time of Hunnish domination", during the 6th and 7th centuries, Pannonian regions were certainly inhabited by Slavs, who were under the Avarian rule.
Principality
During the Frankish war against Avars, the Royal Frankish Annals made mention of a Wonomyrus Sclavus (Vojnomir the Slav or Zvonomir the Slav) active in 795. Eric, Duke of Friuli, sent Vojnomir with his army into Pannonia, between the Danube and Tisza, where they pillaged the Avars' dominions. The next year the Avars were defeated and Frankish power was extended further east, to the central Danube. In the mid-9th century, Lower Pannonia was already inhabited by a Slavic majority (besides "Pannonian Slavs" including Dulebes and possibly some Croats), and Christian Avars were also found in Lower Pannonia in 873.
After the destruction of the Avarian state, Pannonian Slavs came under the Frankish rule. Initially, local Slavic princes were under Frankish suzerainty, within the March of Pannonia, and some of them are known from Frankish primary sources. Prince Ljudevit was mentioned in the Royal Frankish Annals as Duke of Lower Pannonia (), having led an uprising against the Franks (811–822). Their motives aren't known but it's presumed to have been led by the desire for greater autonomy. He was joined by the Carantanians, Carniolans, reportedly Slavs around Salzburg and was supported by (Patriarch of Grado) - which was a substantial threat as their strength partly mirrored the former Avar Khaganate. His stronghold was in Sisak (), former metropolis of ancient Roman province Pannonia Savia. However, the exact boundaries of his principality are uncertain as the term of Lower Pannonian could have implied both the lands between the river Drava and Sava as well as North of them and East of them in the former Roman province Pannonia Secunda (today's Syrmia). Possibly his rule expanded further to the East because in the historical sources is said to have been joined by the tribe of Timočani who lived around Timok Valley (in today's Eastern Serbia). The size of the principality had to be proportional to the resources needed to rebel against the military forces of Borna of Dalmatia and Franks.