Complex sentence: By the 2nd century BC the power of the Odrysian kingdom declined and it became a client state of the Roman Republic but kept its independence until 46 BC, when it was annexed by the Roman Empire, forming the province of Thracia.
Split into simpler sentences: By the 2nd century BC, the power of the Odrysian kingdom declined and it became a client state of the Roman Republic. However, it kept its independence until 46 BC, when it was annexed by the Roman Empire, forming the province of Thracia.