Complex sentence: The origins of the Huns that swept through Europe during the 5th century remain unclear; they may have been connected with the Xiongnu and the later Northern Xiongnu, defeated and dispersed by China some three centuries before.
Split into simpler sentences: The origins of the Huns that swept through Europe during the 5th century remain unclear. Some historians have speculated that they may have been connected with the Xiongnu and the later Northern Xiongnu, which had been defeated and dispersed by China some three centuries before.