Balaton Principality or Lower Pannonia
The Balaton Principality also known as Lower Pannonia was established by the Franks as a frontier march when they destroyed the Avar state in the western part of the Pannonian plain. About 840, Louis the German gave part of the Balaton Principality at Zala River as a fief to the Slavic Prince Pribina who escaped from Moravia during its struggle for independence with the Carolingian Empire. Pribina established himself as prince with the capital in Blatnograd but he ruled the rest of the Balaton Principality from the Raba river to the rivers Drava and Danube including Syrmia as a frontier count.
Pribina was killed as a Frankish vassal in the battle against Great Moravia in 861 and was succeeded by his son Prince Kocel. Like his father, Kocel was initially also a loyal Frankish vassal but he allied himself with Great Moravian Prince, Rastislav against the Carolingians in 869. Pope Hadrian II named Methodius bishop of Sirmium on Kocel’s request and the Balaton Principality became the center of Slavic liturgical and cultural movement. The reaction of the Carolingian authorities and Frankish-Bavarian clergy on the events in the Balaton Principality was bitter. The Balaton Principality returned under Carolingian rule on Kocel’s death in 876, while Archbishopric of Salzburg suppressed the followers of Methodius. However, the Carolingian rule in the Balaton Principality was very short. The territory of the Balaton Principality was invaded and captured by the Hungarians at the end of the 9th century.