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JUSTINIAN AND THE BYZANTINE EMPIRE 151 Asia. Their small boats, however, were soon destroyed, and, convinced that the city could not be taken so long as it remained open by sea, they raised the siege. Two years later Heraclius concluded peace with the Persians, who returned the cross and all their other conquests and cap- tives. But Syria, Mesopotamia, and Egypt had suffered terribly in the long wars and now were taxed heavily to pay for them. What is more, only five years were to pass before they would be exposed to the irresistible expansion of the warlike religion which Mohammed had been founding in Arabia. During these Persian wars the Avars and Slavs had been scouring the Balkan peninsula, destroying towns, carrying off thousands of captives, or settling in the terri- Slavic set- tory which they had desolated. From this period th e m Baikan we may date the beginnings of the present Bal- peninsula kan peoples — Serbs, Croats, and Bulgarians, though the first two names are not met with until the ninth century. Sirmium, the key to the northern half of the peninsula, fell in 582. During the next century the Slavs settled in the depopulated lands south of the Danube, so that seven different racial groups of them were counted between the river and the Balkan Mountains. They also pressed south of that range into Thrace and Macedonia, and thence west into Albania, Dalmatia, and the eastern Alps, and south- ward into Greece. Concerning this great change of popula- tion there is only a single contemporary source of the seventh century, but conclusions are also drawn from the relative thickness with which Slavic place names have sup- planted those of classical geography. It is also supposed that at this time the Slavs left Dacia in such numbers, to migrate south of the Danube, that the previous Roman population came again to preponderate in what is to-day called on that account Roumania. In Greece itself the ancient Hellenic language was to survive, but the popula- tion ceased to be of pure Hellenic descent, and as late as the fifteenth century there were still in Laconia, the southern- most province of Greece, people speaking a Slavic dialect.