The map of the eastern part of Charlemagne's empire a century after his death indicates that the whole region had fallen into certain large divisions ruled over by dukes, who, in Saxony and Bavaria at least, were kings in all but name.[97] Just how these duchies originated is something of a mystery, but two things at least are clear which help to explain their appearance. In the first place, under the weak successors of Louis the German, the old independent spirit of the various peoples, or _stems_, that Charlemagne had been able to hold together, once more asserted itself and they gladly returned to the leadership of their own chiefs. In the second place, they were driven to do this by the constant attacks from without, first of the Northmen and the Moravians, a Slavic people, then of the terrible Hungarian horsemen who penetrated more than once as far west as France. As there was no competent central power to defend the people, it was natural that they should look to their local leaders for help and guidance. [Sidenote: Henry I, 919-936.] These _stem duchies_, as the Germans call them, prevented the German kings from getting a firm hold on their realms. The best that they could do was to bring about a sort of confederation. Consequently, when the German aristocracy chose the strong Henry I, of the ducal house of Saxony,[98] as their king in 919, he wisely made no attempt to deprive the several dukes of their power. He needed their assistance in the task of dealing with the invaders who were pressing in on all sides. He prepared the way for the later subjugation of the Slavs and the final repulse of the Hungarians, but he left to his famous son, Otto I, the task of finally disposing of the invaders and attempting to found a real kingdom. [Sidenote: Otto the Great, 936-973.] The reign of Otto I (936-973), called the Great, is one of the most extraordinary in the history of Germany. He made no attempt to abolish the duchies, but he succeeded in getting all of them into the hands of his sons, brothers, or near relatives, as well as in reducing the power of the dukes. For example, he made his brother Henry duke of Bavaria, after forgiving him for two revolts. His scholarly brother, Archbishop Bruno of Cologne,[99] he made duke of Lorraine in the place of his faithless son-in-law, Conrad, who had rebelled against him. Many of the old ducal families either died out or lost their heritage by unsuccessful revolt. None of them offered a long succession of able rulers. The duchies consequently fell repeatedly into the hands of the king, who then claimed the right to assign them to whom he wished.