Colin McEvedy, whose estimates for ancient cities are much lower than the general consensus, put the population at only 7,000, based on the size of the archaeological site.

[4] Remains of Sirmium stand on the site of the modern-day Sremska Mitrovica, 55 km (34 mi) west of Belgrade (Roman Singidunum).

[5] The city was first mentioned in the 4th century BC and was originally inhabited by the Illyrians and Celts[6] (by the Pannonian-Illyrian Amantini[7] and the Celtic Scordisci[8]).

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.

[10][11] 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.

The eastern part of Illyricum remained a separate prefecture under the East Roman (Byzantine) Empire with its new capital in Thessalonica.

For a short time, Sirmium was the center of the Kingdom of the Gepids, and king Cunimund (r. c. 560 – 567) minted gold coins there.

The city was also the location of the Battle of Sirmium that took place in 1167, where a Roman army dispatched by Manuel I Komnenos decisively defeated the forces of Hungary, turning the latter into a satellite state.

[13] At Glac near Sirmium a palace is being excavated,[14] indicated by the luxurious construction materials coming from all over the Mediterranean, such as red and green porphyry from Egypt and the Peloponnese, and marble from Tunisia, Greece and Italy.

[citation needed] During work on the new Sremska Mitrovica trade center in 1972, a worker accidentally broke into an old Roman pot, about 2m deep, over the site of an old Sirmium settlement.

The presence of the arena has clearly affected the layout of the present town (Sremska Mitrovica is today about 2–4m above the ground line of the former Sirmium settlement).