The concept of Germany as a distinct region in central Europe can be traced to Roman commander Julius Caesar, who referred to the unconquered area east of the Rhine as Germania, thus distinguishing it from Gaul (France), which he had conquered. The victory of the Germanic tribes in the Battle of the Teutoburg Forest (AD 9) prevented annexation by the Roman Empire. Following the fall of the Roman Empire, the Franks conquered the other West Germanic tribes. When the Frankish Empire was divided among Charlemagne's heirs in 843, the eastern part became East Francia. In 962, Otto I became the first emperor of the Holy Roman Empire, the medieval German state.