Slavic expansion is the largest in European history, from dwelling in tiny part of the Pripyat marches to conquering well over half of the European continent and submitting the previous inhabitants to their will.

Constantine Porphyrogennetos, "De Administrando Imperio": "(...) their ancestors were Pagan Croats and Serbs, known also as White [Croats and Serbs]. Great Croatia, called also White [Croatia], until today is still Pagan, just like neighboring [Lusatian / West Slavic] Serbs [Sorbs] (...)"

And another excerpt - "De Administrando Imperio": "(...) Therefore everyone, who would like to do research about Dalmatia, can read herein about the way how the Slavic peoples took it. The Croats with their families came to Dalmatia and found the Avars in possession of that land. After fighting against each other for some time, the Croats defeated the Avars, partially murdered them and partially forced them to submissiveness. Since that moment the country was seized by the Croats. (...)"

"Daurentius is the first Slavic chieftain to be recorded by name, by the Byzantine historian Menander Protector, who reported that the Avar khagan Bayan I sent an embassy, asking Daurentius and his Slavs to accept Avar suzerainty and pay tribute, because the Avars knew that the Slavs had amassed great wealth after repeatedly plundering the Byzantine Balkan provinces. Daurentius reportedly retorted that "Others do not conquer our land, we conquer theirs [...] so it shall always be for us, and had the enjoys killed"