Sirmium was a city in ancient Roman Pannonia. 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 Lower Pannonia. 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 SecundaProvince. Sirmium was located on the Sava river, on the site of modern Sremska Mitrovica in Vojvodina province, northern Serbia. The Republic of Serbia declared its site an Archaeological Sites of Exceptional Importance in 1990. The modern region of Syrmia (Srem) is named after it.
Sirmium had 100,000 inhabitants and was one of the biggest cities of its time. Colin McEvedy, however, puts the population at only 7.000, based on the size of the archaeological site. Ammianus Marcellinus called it "the glorious mother of cities".