Sirmium was a city in the Roman province of Pannonia, located on the Sava river, on the site of modern Sremska Mitrovica in the Vojvodina autonomous provice of 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. Sirmium purportedly had 100,000 inhabitants and was one of the largest cities of its time. Colin McEvedy, whose estimates for ancient cities are much lower than the general consensus, however, put the population at only 7,000, based on the size of the archaeological site. The amount of grain imported between 1 AD and 400 AD was enough to feed 700,000 to 1 million people. History Remains of Sirmium stand on the site of the modern-day Sremska Mitrovica, west of Belgrade (Roman Singidunum) and away from Kostolac (Roman Viminacium). Archaeologists have found traces of organized human life on the site of Sirmium dating from 5,000 BC. The city was first mentioned in the 4th century BC and was originally inhabited by the Illyrians and Celts (by the Pannonian-Illyrian Amantini and the Celtic Scordisci). The Triballi king Syrmus was later considered the eponymous founder of Sirmium, but the roots are different, and the two words only became conflated later. The name Sirmium by itself means "flow, flowing water, wetland", referring to its close river position on the nearby Sava. With the Celtic tribe of Scordisci as allies, the Roman proconsul Marcus Vinicius took Sirmium in around 14 BC. In the 1st century AD, Sirmium gained the status of a Roman colony, and became an important military and strategic center of the Pannonia province. The Roman emperors Trajan, Marcus Aurelius, and Claudius II prepared war expeditions in Sirmium. In 103, Pannonia was split into two provinces: Pannonia Superior and Pannonia Inferior; Sirmium became the capital city of the latter. In 296, Diocletian reorganized Pannonia into four provinces: Pannonia Prima, Pannonia Valeria, Pannonia Savia and Pannonia Secunda, with Sirmium becoming the capital of Pannonia Secunda.