The Bulgars, like the Huns and the Avars who preceded them, and like the Magyars and the Turks who followed them, were a tribe from eastern Asia, of the stock known as Mongol or Tartar. The tendency of all these peoples was to move westwards from Asia into Europe, and this they did at considerable and irregular intervals, though in alarming and apparently inexhaustible numbers, roughly from the fourth till the fourteenth centuries. The distance was great, but the journey, thanks to the flat, grassy, treeless, and well-watered character of the steppes of southern Russia which they had to cross, was easy. They often halted for considerable periods by the way, and some never moved further westwards than Russia. Thus at one time the Bulgars settled in large numbers on the Volga, near its confluence with the Kama, and it is presumed that they were well established there in the fifth century. They formed a community of considerable strength and importance, known as Great or White Bulgaria. These Bulgars fused with later Tartar immigrants from Asia and eventually were consolidated into the powerful kingdom of Kazan, which was only crushed by the Tsar Ivan IV in 1552. According to Bulgarian historians, the basins of the rivers Volga and Don and the steppes of eastern Russia proved too confined a space for the legitimate development of Bulgarian energy, and expansion to the west was decided on. A large number of Bulgars therefore detached themselves and began to move south-westwards. During the sixth century they seem to have been settled in the country to the north of the Black Sea, forming a colony known as Black Bulgaria. It is very doubtful whether the Bulgars did take part, as they are supposed to have done, in the ambitious but unsuccessful attack on Constantinople in 559 under Zabergan, chief of another Tartar tribe; but it is fairly certain that they did in the equally formidable but equally unsuccessful attacks by the Slavs and Avars against Salonika in 609 and Constantinople in 626.