After the collapse of the revolt of Ljudevit Posavski, Lower Pannonia fell into a complete vassal relationship with the Frankish emperor. However, in 827, the Bulgarian prince Mortag moved against the Franks, and the Pannonian duke Ratimir joined him. Having defeated the Franks, the Bulgarians, who had already conquered Srijem, established a loyal government in the newly conquered areas, and Ratimir seemed to be in a kind of vassal relationship with the Bulgarians. Such an outcome prompted Emperor Louis to remove the Friulian Margrave Balderic from his position in 828, and he divided the Friulian Margraviate into four parts. In 828 and 829, the Franks, led by Ludwig, son of Louis and future German king, fought against the Bulgarians and Croats, but without success, which probably contributed to the disintegration in Francia, when Ludvik’s sons rose up against their father and divided the empire. Ludwig of Germany, now as king, again waged war in 838 against Ratimir, who escaped. The Franks then regained part of the lost territory, but they did not succeed in doing so until 845, when they reconquered all of Sclavonia and Srijem.
In the following decades, the lands between the Drava and the Sava rivers recognized Frankish supremacy, and they were ruled by the Slavic dukes Pribina and later Kocelj. Pannonian Croatia continued to serve as a Bulgarian-Frankish battlefield during the war for the Italian heritage of Carolingians. It was also the period of the first organized colonization, which, however, could not have had much intensity. There was a lot of desolate land, but there was still no great demand for it in Europe, and the border was not peaceful enough. After his death in the war with the Moravians, Pribina was succeeded in 861 by his son Kocelj. Further development proceeded mostly peacefully until the time of the Magyar (Hungarian) invasions of Pannonia.
At that time, part of Lower Pannonia was ruled by the Great Moravian Duke Svatopluk, who came into conflict with the Franks. The then duke of Pannonian Croatia, Braslav, in order to secure himself before the powerful Svatopluk, entered into a stronger connection with the Franks. After the death of the Frankish emperor Charles III the Fat (887), Arnulf of Carantania comes to power. He resumed hostilities against Svatopluk, and Braslav became involved in that war on Arnulf’s side. Arnulf also called for help from the Hungarians who were then north of the Carpathians, and Svatopluk was defeated. After the Bulgarian invasions of their territories, the Hungarians settled in the central part of the Pannonian Plain around 896, and in the same year Arnulf handed over to Braslav the former Kocelj’s land to defend it from the Hungarians, but he did not succeed. On the contrary, the Hungarians penetrated into Pannonian Croatia itself, and through it to Coastal Croatia and even northeastern Italy. It seems that Braslav, as the last prince of Pannonian Croatia, died in 896 in battles against the Hungarians.
At the beginning of the 10th century, the strength of Coastal duchy of Croatia grew stronger, until duke and then king Tomislav annexed Pannonian Croatia to his rule. This act began the further rapprochement of the two principalities that would later form medieval Croatia. Very little is known about the area of the Pannonian principality in the period from the beginning of the 10th to the end of the 11th century. It is known that the church belonged to the Split metropolis and formally the Sisak diocese, which seems to have existed only on paper, but was really under the jurisdiction of the Croatian court bishop. The archeological finds of that area at that time belong to the Pannonian culture called Bjelo brdo, although there are also old Croatian finds, but it seems to be mostly from the 11th century.