and 6th centuries CE (thought to be in conjunction with the movement of peoples from Siberia and Eastern Europe: Huns, and later Avars and Bulgars) started the great migration of the Slavs, who settled the lands abandoned by Germanic tribes fleeing the Huns and their allies: westward into the country between the Oder and the Elbe-Saale line; southward into Bohemia, Moravia, much of present-day Austria, the Pannonian plain and the Balkans; and northward along the upper Dnieper river. It has also been suggested that some Slavs migrated with the Vandals to the Iberian Peninsula and even North Africa. Around the 6th century, Slavs appeared on Byzantine borders in great numbers. Byzantine records note that Slav numbers were so great, that grass would not regrow where the Slavs had marched through. After a military movement even the Peloponnese and Asia Minor were reported to have Slavic settlements. This southern movement has traditionally been seen as an invasive expansion. By the end of the 6th century, Slavs had settled the Eastern Alps regions. Pope Gregory I in 600 CE wrote to Maximus, the bishop of Salona (in Dalmatia), in which he expresses concern about the arrival of the Slavs: "Et quidem de Sclavorum gente, quae vobis valde imminet, et affligor vehementer et conturbor. Affligor in his quae jam in vobis patior; conturbor, quia per Istriae aditum jam ad Italiam intrare coeperunt." ("I am both distressed and disturbed about the Slavs, who are pressing hard on you. I am distressed because I sympathize with you; I am disturbed because they have already begun to arrive in Italy through the entry-point of Istria.") Middle Ages When Slav migrations ended, their first state organizations appeared, each headed by a prince with a treasury and a defense force. In the 7th century, the Frankish merchant Samo supported the Slavs against their Avar rulers and became the ruler of the first known Slav state in Central Europe, Samo's Empire. This early Slavic polity probably did not outlive its founder and ruler, but it was the foundation for later West Slavic states on its territory. The oldest of them was Carantania; others are the Principality of Nitra, the Moravian principality (see under Great Moravia) and the Balaton Principality. The First Bulgarian Empire was founded in 681 as an alliance between the ruling Bulgars and the numerous Slavs in the area, and their South Slavic language, the Old Church