Consolidation of the eastern part of the Carolingian Empire

Know why the eastern part of the Carolingian Empire was consolidated.
                   The Carolingian Empire was divided into three parts shortly after Charlemagne's death. In 843 C.E., the Frankish lands were broken into: western, central, and eastern areas. This made the kingdoms weak. The eastern part of the Carolingian Empire was continuously under attack by Vikings and Muslims. In the mid-10th century, the eastern portion was consolidated under new rule: the Saxon-German emperors know as the Ottonians.