Complex sentence: When the Frankish Empire was divided up among the three sons of Louis the Pious in 843, Speyer fell under the reign of Louis the German who was given the eastern Frankish territories, East Francia, which was to become the Kingdom of Germany.
Split into simpler sentences: When the Frankish Empire was divided up among the three sons of Louis the Pious in 843, Speyer fell within the portion of Louis the German. Louis received the eastern Frankish territories, East Francia, which was to become the Kingdom of Germany.