Complex sentence: The Uigur Kaganate was routed by their warlike neighbours Kirghiz, who destroyed the advanced culture of the Uigur, and the cultural development of the country was thrown back to the primitive stage.
Split into simpler sentences: The Uigur Kaganate was routed by their warlike neighbours Kirghiz, who destroyed the advanced culture of the Uigurs. The cultural development of the country was thrown back to the primitive stage.