_Serbia._--Invading tribes of Don Cossacks began to come in great numbers to the Balkan Peninsula in the sixth century. In the seventh century they were encouraged by the Greek Empire to settle in Serbia, on condition of paying tribute to Constantinople. They set up a kind of aristocratic republic of a Slav type. In the ninth century they began to fight with the neighbouring and kindred Bulgarians. Early in the tenth century (A.D. 917) the Bulgarians almost effaced Serbia from the map; but the Serbs recovered after half a century, only to come shortly afterwards under the sway of the Greeks. In the eleventh century the Serbians held a very strong position and were able to harass the Greek Empire at Constantinople. They entered into friendly relations with the Pope of Rome, and for some time contemplated following the Roman rather than the Eastern Church. In the twelfth century King Stephen of Serbia was a valued ally of the Greek Empire against the Venetians. He established Serbia as a European "Power," and the Emperor Frederick Barbarossa visited his court at Belgrade. This king was the first of a succession of able and brave monarchs, and Serbia enjoyed a period of stable prosperity and power unusually lengthy for the Balkans. Except for the strife between the Eastern and Roman Catholic Churches for supremacy in Serbia, the nation was at peace within her own borders, and enjoyed not only a military but an economic predominance in the Balkans. Mining and handicrafts were developed, education encouraged, and the national organisation reached fully to the average standard of European civilisation at the time. By 1275 the Serbs were the chief power in the Balkans. They defeated the Greeks, marched right down to the Aegean and reached the famous monastery of Mount Athos, to which the first King Stephen (Nemanya) had retired in 1195 when he abdicated.