Complex sentence: The Tibetans had benefited lot during Tang properous time when Tang aid was frequent and during the Anshi Rebellion, the kingdom of Tibet was in its height of power by treacherously supporting the Rebels.
Split into simpler sentences: The Tibetans had benefited a lot during Tang's properous period when Tang aid was frequent. During the Anshi Rebellion, the kingdom of Tibet was in its height of power by treacherously supporting the Rebels.