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 Slavic inhabitants of the region lived under the rule of the Pannonian Avars. By the beginning of the 9th century, after the Avar Wars, the Avar Khaganate was and replaced by the Frankish Empire, which lasted until the Magyar conquest (c. 900).
Within the Frankish administrative system, the March of Pannonia was created. Direct Frankish rule was exercised in Upper Pannonia, while local Slavic princes governed the Principality of Lower Pannonia under Frankish suzerainty. 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 split the Slavs in the region into the West Slavs and the South Slavs.
Background
Roman rule in Pannonia collapsed during the 5th century, and was replaced by the rules of Huns, Ostrogoths and Lombards. During the reign of Byzantine Emperor Justin II (565–578), and following the Lombard-Gepid War in 567, Avars invaded Pannonia and conquered almost all of the Pannonian Plain. The first indications of the presence of Proto-Slavic groups in Pannonia occur in the 5th century during Hunnic rule. In the 6th and 7th centuries, the inhabitants of Pannonia were certainly Early Slavs. 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.
Principality of Lower Pannonia
During the Avar wars, the Royal Frankish Annals mentioned 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 rivers, where they pillaged the Avars' dominions. The next year the Avars were defeated and Frankish power was extended further east, to the central Danube.
After the destruction of the Avarian state, Pannonian Slavs came under 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 (Latin: Liudewiti, ducis Pannoniae inferioris). He led an uprising against the Franks from 811 to 822. The motives for the uprising are unknown, but are presumed to include a desire for greater autonomy. He was joined by the Carantanians, Carniolans, and reportedly by Slavs around Salzburg. Fortunatus II, Patriarch of Grado, supported him. Their combined strength was comparable to the former Avar Khaganate.
Ljudevit's stronghold was in Sisak (Latin: Siscia), former metropolis of ancient Roman province Pannonia Savia. However, the exact boundaries of his principality are uncertain as the term "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). It is possible that his rule extended further east, because historical sources state that the tribe of Timočani who lived around Timok Valley (in today's eastern Serbia) joined him. He rebelled against the military forces of Borna of Dalmatia and the Franks.
After Ljudevit's death and failure of his uprising, in 827 the Bulgars under Great Khan Omurtag invaded and conquered Lower Pannonia and parts of Frankish territories to the north. They also installed their own governors. The Bulgarian-Frankish conflict was probably spurred over the control of the Timočani and Danubian Abodrite tribes. Louis II of Germany made a counterattack in 828, and eventually the March of Friuli was divided into four counties. One of them probably was the early Duchy of Croatia (which also expanded upon the territory of Sisak). Pannonia again became part of the Pannonian March. Both marches were vassals to East Francia. The next year the Bulgars attacked again unsuccessfully, although the territory of Pannonia most probably lost its eastern part to the First Bulgarian Empire.
In 838 Ratimir, a local Slavic prince, emerged as the new ruler of Lower Pannonian regions, around the rivers Drava and Sava. He probably ruled the eastern areas of Pannonia and was a governor of the Bulgarian Empire. Pribina, former prince of the Principality of Nitra, fled to him after being expelled by Mojmir I of Moravia. In the same year Frankish count Radbod of the East March deposed Ratimir and strengthened Frankish rule in Lower Pannonia. Ratimir fled the land, and the Franks instated Slavic prince Pribina as the new ruler of Lower Pannonia. Pribina (d. 861) was succeeded by his son, prince Kocel. During the rules of Pribina and Kocel, the capital of the Principality of Lower Pannonia was Mosapurc (Mosapurc regia civitate), also known in Old Slavonic as Blatnograd (modern Zalavár near Lake Balaton). The polity was a vassal principality of the Frankish Empire, or according to others, a frontier county (Latin: comitatus) of the Eastern Frankish Kingdom. It was initially led by a dux (Pribina) and later by a comes (Kocel) who was titled as "Count of the Slavs" (Latin: Comes de Sclauis). Their authority stretched towards the northwest up to the Rába river and Ptuj, and to the southeast up to the Baranya region and the Danube river. During the time of Kocel, Byzantine missionary Methodius was active in Lower Pannonia. Pope John VIII's letter to a dux Mutimir of uncertain identity, commonly considered to be Mutimir of Serbia is dated to be in this period. The letter concerns the formation of the Diocese of Pannonia with its seat in Sirmium Methodius became the archbishop of the diocese at Kocel's request. (see also Archbishopric of Moravia).
The course of events at the end of the 9th century is unclear. Although Lower Pannonia was still under Frankish influence, Svatopluk I of Moravia emerged as a new threat. Braslav was the last dux of Lower Pannonia between at least 884 and 896. He ruled between the Drava and Sava under the overlordship of Arnulf of Carinthia. He participated in the Frankish–Moravian War, and in 895 or 896 Arnulf handed over Pannonia to him in order to secure the Frankish frontier against a new threat - the Hungarians who had conquered Great Moravia. However, the Hungarians subsequently overran all of the Pannonian Basin.
Aftermath
Following the rise of the Principality of Hungary in the mid 890s, and especially after Battle of Pressburg (907), no further Slavic rulers were recorded in the regions of Lower Pannonia. The Hungarian conquest separated the West Slavs from the South Slavs, influencing the formation of new Slavic identities. Some Moravian Slavs also fled to the Duchy of Croatia. The new border between Croatia and Hungary was apparently north of the town of Sisak, based on a recent archaeological finding of a "knez from Bojna" near Glina. The diocese with its seat in Sisak was offered at the council of Split (928) to bishop Gregory of Nin, which could have only been possible if it was within the borders of the Kingdom of Croatia. In the mid 920s, Tomislav of Croatia expanded his rule to some Lower Pannonian territories, between Sava and Drava, adding them to the Croatian kingdom.
Until the end of the 11th century, its western border was also contested by the Holy Roman Empire,. At the same time, Hungarians and Bulgarians contested for the southeastern Pannonian regions (Syrmia). The borders between the Croatian and Hungarian states in the 10th and 11th century are disputed in modern historiography. Croatian historian Ferdo Šišić and his followers assume that Tomislav of Croatia had ruled most of the area inhabited by Croats, including southern Pannonian regions (Slavonia), while the Hungarian historians Gyula Kristó, Bálint Hóman and János Karácsonyi think that the area between Drava and Sava rivers belonged neither to Croatia nor to Hungary at the time. Nada Klaić stated that she could not preclude this opinion, because the generic name "Slavonia" (lit. the land of the Slavs) may have implied so.
The population of the region grew and churches and administrative organizations were formed, including the diocese of Zagreb (1094). Even after Croatia entered a personal union with the Kingdom of Hungary, Lower Pannonia retained partial autonomy having governor titled as Ban of Slavonia.
Archaeology
Many cemeteries were found in Lower Pannonia, showing that the region had a large population in the 11th century. The inhumation practices and rituals of the population of Pannonia varied with culture and ethnicity. Even after the Frankish defeat of Avars and the process of Christianization some pagan practices and rituals were retained, such as cemeteries being arranged in rows, feasting at the funeral or steppe burial rites with horses and equipment. Many new settlements were founded around ancient towns. One of them, Sisak, was even the seat of a Christian diocese. The native and mainly sedentary Slavic population assimilated the Avars, forming the Avaro-Slavic Middle-Danube culture. Sedov considered that those Slavs were a mixture of Sclaveni of Prague-Korchak culture and mostly Antes of Penkovka and Ipotesti–Candesti culture, with some Martinovka culture artifacts. According to M. Guštin and L. Bekić, radiocarbon dating confirmed the origin of the artifacts to the late 6th and early 7th century. Although the Pannonian and near Alpine Slavs material culture had features of both Korchak and Penkovka-type, Korchak-type predominates, with parallels in northern Slovenia, Austria and Hungary, indicating migration to northern Croatia through the Moravian Gate between eastern Austria and western Hungary, but not excluding another migration wave from Lower and Middle Danube, upstream of the river Sava and Drava. Later they also assimilated Hungarians, but eventually through the administrative system were linguistically assimilated by the Hungarians themselves. In the 10th century, a so-called Bijelo Brdo culture was formed due to interaction with the Hungarians, located in the area of Middle Danube. In the late 10th and early 11th centuries contacts between Bijelo Brdo, old Croatian-Dalmatian, Carinthian-Köttlach and old Hungarian cultures are observed in modern-day Northern Croatia.
According to the craniometrical measurements and archaeological findings, early Croats did not initially settle in Lower Pannonia and their relationship with Pannonian Slavs was more political rather than ethnic. Others argue that the "Bijelo Brdo and Vukovar cemeteries can hardly be regarded evidence of a pre-Croatian Slavic population in northern Croatia" and they rather "represent a population fleeing the Magyars" during the 10th century".
Language
The primary sources for studying the Pannonian Slavic language are the oldest layer of Slavic loanwords in Hungarian, the Kiev Missal (and possibly the Freising Fragments), and scarce data from toponymy and onomastics. Dialectal features of Slavic varieties spoken in areas bordering modern Hungary are also used in the analysis.
The first linguist to seriously analyze Pannonian Slavic was Eugene Helimski in 1988. He regarded it as a koiné later absorbed by Hungarian as a substratum. Helimski suggests that a dialect continuum between West Slavic and South Slavic existed across Pannonia in the Early Middle Ages. According to Alexei S. Kassian (2024), analysis of the earliest Slavic borrowings in Hungarian clearly shows that the donor language belonged to the West Slavic group.