Sirmium was a city in the Roman province of Pannonia, located on the Sava river, on the site of modern Sremska Mitrovica in northern Serbia. First mentioned in the 4th century BC and originally inhabited by 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 proclaimed one of four capitals of the Roman Empire. It was also the capital of the Praetorian prefecture of Illyricum and of Pannonia Secunda. The site is protected as an Archaeological Site of Exceptional Importance. The modern region of Syrmia (Srem or Srijem) was named after the city.