Complex sentence: Under Emperor Diocletian, Marcianopolis (Devnya) became the centre of the Roman province of Moesia Secunda of the Diocese of Thrace; during Emperor Valens' wars with the Goths (366-369), this city was temporary capital of the empire.
Split into simpler sentences: Under Emperor Diocletian, Marcianopolis (Devnya) became the centre of the Roman province of Moesia Secunda of the Diocese of Thrace. During Emperor Valens' wars with the Goths (366-369), this city was temporary capital of the empire.