Complex sentence: Ruling the Eastern Empire for nearly 20 years from 457 to 474, Leo proved to be a capable ruler, overseeing many ambitious political and military plans, aimed mostly for the aid of the faltering Western Roman Empire and recovering its former territories.
Split into simpler sentences: Ruling the Eastern Empire for nearly 20 years, Leo proved to be a capable ruler. He oversaw many ambitious political and military plans, aimed mostly for the aid of the faltering Western Roman Empire and recovering its former territories.