Complex sentence: The Kumo Xi and the Khitans were united in a tribal Kumo Xi entity until 388 AD, when the previously defeated Kumo Xi leaders insisted on fighting against with the proto-Mongolic Northern Wei dynasty.
Split into simpler sentences: The Kumo Xi and the Khitans were united in a tribal Kumo Xi entity. In 388 AD, the previously defeated Kumo Xi leaders insisted on fighting against with the proto-Mongolic Northern Wei dynasty.