Complex sentence: The indigenous population of Thrace was a people called the Thracians, divided into numerous tribal groups.The region was controlled by the Persian Empire at its greatest extent, and Thracian soldiers were known to be used in the Persian armies.
Split into simpler sentences: The indigenous population of Thrace was a people called the Thracians, divided into numerous tribal groups. A large part of the region was at times controlled by the Persian Empire at its greatest extent, and Thracian soldiers were known to be used in the Persian armies.