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JUSTINIAN AND THE BYZANTINE EMPIRE 139 and strengthening the Empire was the hostility of the Persian Kingdom on his eastern frontier. Wars which were none of his seeking and which lasted from 524 to 532, from 540 to 545, and from 549 to 562, ended by his agreeing to pay Persia an annual tribute. Meanwhile he had been forced to draw away so many troops from the northern frontiers for these Persian wars and for the long-drawn-out conquests of Africa and Italy, that the Huns, Slavs, and Bulgars were able to make incursions across the Danube on an average of one in four years for the reign. In the end they were always driven back, but sometimes got as far as the Isthmus of Corinth or the environs of Constantinople In his old age Belisarius gained his last laurels by repulsing a great invasion of the Huns in 558. The original Bulgars were nomads like the Huns and fol- lowed them into the Pontus Steppe at a somewhat later date. They first appeared south of the Danube toward the end of the fifth century. As the Huns a century before had conquered many German tribes and driven others into the Roman Empire, so now the Bulgars carried the Slavs with them in frequent raids across the Danube. Though originally the masters, the Bulgars were eventually to adopt the language and customs of the Slavs, and fuse with them into the Bulgarian nation that we know to-day. The earH history of the Slavs is uncertain. They are classed a? x3f Alpine race, and their closest racial affilia- tio; * seems to be with the Celts ; they speak Ian- >_, „ 1 The Slavs guages of the Indo-European group. They in- clude Letts and Lithuanians near the Baltic Sea as well as the Russians and the Slavs south of the Danube. Several centuries before our era the Germans had pushed them back east of the Vistula, but in the early centuries of our era the Slavs appear to have multiplied rapidly in numbers and to have expanded widely over eastern Europe. They were an agricultural peasantry, inferior to the Germans, however, in their vegetarian diet and lack of domestic animals to aid their labors. Also political and social institutions were little