Sirmium was a city in Pannonia, an ancient province of the Roman Empire. First mentioned in the 4th century BC and originally inhabited by the Illyrians and Celts, it was conquered by the Romans in the 1st century BC and subsequently became the capital of the Roman province of Pannonia Inferior. In 294 AD, Sirmium was pronounced one of the four capitals of the Roman Empire. It was also the capital of the Praetorian prefecture of Illyricum and of Pannonia Secunda Province.