The principality was one of the several Slavic states and groups connecting the areas inhabited by Slavs before they were divided into the northern and the southern Slavs by the conquests of the Franks, the arrival of the Magyars in Pannonia, and later by the expansion of the Romanians.
The Slavic people of that time were weakly differentiated, speaking closely related dialects of the same common language. The inhabitants of the Balaton Principality were most probably closely related to each of neighboring Slavic people: Great Moravians (Moravians and Slovaks) to the north, Karantanians (Slovenes) to the west, Pannonian Croats to the south, Serbs to the south-east and Severans to the east, providing the bridge between those Slavic states and tribal unions.
The Slavic inhabitation of Pannonia started in the 5th century after the fall of the Hunic tribal union, but the most intense immigration occurred during the second half of the 6th century, as a part of the Avar tribal union (Avar Khaganate). After the coordinated attack by Franks (led by Charles the Great) and Bulgars (led by khan Krum) which defeated the Avars, the liberated Slavs of Pannonia started organizing semi-independent political units.
In the course of the creation of (see) Great Moravia in 833 to the north of the Danube, Pribina (Priwina), until then the Prince of the Nitrian principality (Slovakia), was expelled from his country by the prince of the Moravian principality, and later in 839 or 840 founded the Balaton Principality (whose Slavic name means "Principality (Duchy) of the Muddy lake (or river)"). Its capital was the Blatnograd (Blatnohrad, later called Mosapurc), a fortified city built at the Zala river (Zala in Hungarian, in Slavic languages "Blatna" or similar forms meaning Muddy river) between the small and large Balaton lakes (Balaton in Hungarian, in Slavic languages Blatno / Blatenské jazero or similar forms meaning Muddy lake)
During the reign of Pribina's son, prince Kocel (Gozil, Koceľ, Kocelj) (861-876), in the summer of 867, it provided short-term hospitality to brothers Cyril and Methodius on their way from Great Moravia to the pope in Rome to justify the use of the Slavonic language as a liturgical language. They and their disciples turned Blatnograd into one of the centers that spread the knowledge of the new Slavonic script (Glagolitic alphabet) and literature, educating numerous future missionaries in their native language.
The principality was founded as a vassal of the east-Frankish (i.e. German) kings, but later it started resisting the influence of German feudal lords and clergy, trying to organize an independent Slavic archdiocese. Eventually, after Kocel's death in 876, it was again made part of the Carinthia March of the East Frankish Empire, and soon afterwards, in 901 it was conquered by the Magyars, who terminated the remaining elements of Slavic self-organization.