With their fellow Slavs more to the south, the Czechs of [Sidenote: Subjection of Bohemia.] Bohemia, the Franks had comparatively few relations. The vast uninhabited tract of forest and mountain called the Böhmerwald seems to have long kept them apart. But in 805-6 the king sent against them his son and namesake Charles the Younger, who twice wasted all the valley of the upper Elbe, and finally compelled the chiefs of the Czechs to acknowledge their dependence on the Frankish empire by paying tribute. South of Bohemia, along the Danube and the Raab and Leithe, the realms of Charles bordered on the Tartar tribe of the Avars, ancient enemies both of the Lombards and of the emperors of Constantinople. The Avars had of late years fallen on evil times. [Sidenote: War with the Avars.] They were vexed with civil wars so much that none of their princes any longer ruled the whole race, or could call himself by the title of Chagan, the old name of their supreme ruler. Yet, though wasted by their own dissensions, and by the revolts of the Slavonic tribes who were their vassals, the Avars could not keep from their old habit of making descents on their neighbours. They drew down their doom on themselves by invading, in 788, at once the Lombard march of Friuli and the vassal duchy of Bavaria. When next he had leisure, two years later, Charles planned an invasion of their land on the largest scale. He himself marched down the Danube with an Austrasian and Saxon army, burst through the long line of fortifications with which the Avars had strengthened their border, and wasted their lands as far as the Raab. At the same moment a great Lombard host entered the valley of the Drave, pushed into the heart of Pannonia, beat the Avars in the field, and stormed their great circular camps. The complete subjection of the whole tribe would have followed in the next year if Charles had not been called away by a Saxon revolt, which kept him employed during the two next campaigning seasons. The king himself never again took the field against the Avars, but his son Pippin and Eric duke of Friuli continued the war on his behalf. Twice they captured the great ‘ring,’ or royal camp, between Danube and Theiss, the central stronghold of the Avar race, and sent its spoils to Aachen in such quantities that Charles was able to send Avaric trophies as gifts to all his friends, even to such distant kings as Offa of Mercia. [Sidenote: The Avars subdued.] At last the spirit of the Avars was so much broken that their chiefs, or ‘Tuduns,’ came of their own accord to Aachen to do homage to Charles, and offered to receive Christianity. Their submission was accepted. The king appointed one of them to rule the whole race as his vassal, and bade him assume the ancient title of Chagan (805). This prince was baptized by the name of Abraham, paid a regular tribute to the Franks, and kept his subjects for the future from the dangerous temptation of meddling with the Lombard or Bavarian border.