Pannonian Croatia is the name for the duchy formed between the Drava River in the north and the Sava and Kupa in the south, in the area of today’s northern Croatia, in the southern parts of the Pannonian Plain. In the sources this area is usually called simply Pannonia, and the people are usually called in general Slavs (Latin: Sclavi). The Pannonian duchy developed in the area of the former Roman provinces of Sava Pannonia and Second Pannonia.
After the Romans, Pannonia came under the rule of the Eastern Goths (453), the Lombards (527) and the Avars (568). As early as the 6th century, Slavs settled in the area of Pannonia. When Charlemagne destroyed the Avar state in the late 8th century, the Slavs remained in Pannonia. At the end of the 9th century, the Hungarians penetrated across the Carpathians into the Danube lowlands and suppressed the Slavs, occupying Pannonia Valeria and the First Pannonia, except for its northwestern part, which was held by the Germans. Sava Pannonia and the Second Pannonia remained in the possession of the Slavs. Since then, the name Pannonia has been gradually lost.
Many rulers of this principality are not known either. It is known that from 791 to 810 the ruler was Vojnomir. But there are still many unknowns about him. Although he is still often referred to as a duke, there is no such title in the sources. He is simply called a Slav. In addition, the correct form of his name is questionable, because in Frankish sources it is mentioned under the name Uuonomiro or Uuonomyro. Some also think that he was not the ruler of the Pannonian Croats, but of those who had immigrated to Istria under the leadership of Duke Ivan. Other historians are convinced of Vojnomir’s rule in Pannonian Croatia or of his Carinthian origin, which is especially prevalent among Slovenian historians. In 791 Charlemagne began a campaign against the Avars, and the following year, 792 Vojnomir submitted to Charlemagne and promised him help in the fight against the Avars. Having ended the wars in Saxony (792-796), Charles was finally able to turn to the Avars, and the war against them lasted from 796 to 799. In that war, the Pannonian Croats also took part on the Frankish side, and after the final victory which the Franks and Croats the immediate Avar danger disappeared. It is possible that Constantine Porphyrogenitus in his work „De administrando imperio“ refers to this victory, when he speaks of the defeat inflicted by the Croats on the Avars („De administrando imperio“, page 31). The Franks then founded two marks on the eastern borders of their kingdom: the northern (modern Austria), and the southern Friuli.
At the beginning of the 9th century, Pannonian Croats began to abandon their pagan customs and embrace Christianity. This is evidenced by numerous archaeological finds, especially Old Croatian graves in which from that period there are no objects that were buried with the deceased. Of course the conversion to Christianity was gradual and lasted for several centuries.
After the death of Charlemagne, his son Louis I the Pious (814-840) became emperor, who did not reach his father with energy, so the great power in the empire was seized by the nobles. Among them was the Friulian Margrave Kadolah, who with his army penetrated the area of Pannonian Croatia and plundered there. At that time, the prince of Pannonian Croatia was Ljudevit Posavski (810 – 823), known in Frankish chronicles as “… Liudewitus, dux Pannoniae inferioris …” (Ljudevit, duke of Lower Pannonia) who in 818 sent his envoys to the emperor Louis to complain of Kadolah’s actions. But the envoys return without their problems being taken seriously, which encouraged Ljudevit to resist the Franks. This is how the first conflict took place, but Kadolah did not achieve any success, so he returned to Friuli and soon died. In July 819, Emperor Louis held a Council in Ingelheim, which was attended by envoys of duke Ljudevit, to offer peace to the Franks, but the emperor did not accept it.
The duke of Coastal Croatia, Borna, had already joined the Franks, while the Carantans and Timocans, (both Slavs), sided with Ljudevit. As early as the autumn of 819, the new Friulian Margrave Balderic set out with his army for Carantania, where he defeated Ljudevit at the Battle of the Drava. At the same time, duke Borna moved north, but on the day of the battle near the river Kupa, a large part of his army sided with Ljudevit and Borna withdrew. As early as December of the same year, Ljudevit broke into the south against Borna, whose army closed in on the forts and attacked the Pannonian army from them and from the mountains, so the latter had to retreat to the north. In these battles, Ljudevit lost about 3,000 soldiers, over 300 horses and a lot of food.
After the Council of Aachen in 820, in which duke Borna also took part, in the spring of the same year a new campaign against Ljudevit began from three directions: the northern Frankish army from Bavaria, along the Danube valley, and penetrated across the Drava towards Pannonian Croatia; the southern army from Friuli, across the Alps; the third army from Tyrol via Carinthia. The first two armies were stopped by Ljudevit’s forces, the first on the Drava and the second in the Alps. Before the third, he retreated towards Croatia, which then opened the way for the others. After penetrating the area between the Sava and the Drava, the Franks were left with only to plunder, because Ljudevit did not start the battle there. The same thing happened the following year, 821. In October of that year, at the Council of Diedenhofen, the Frankish generals reported to the emperor that Ljudevit did not want to engage in battle. It was not until 822 that Ljudevit left Sisak and fled before the army from Friuli to the south, and the following year further towards the sea, where he was killed.