Sirmium (sûrˈmēəm) [key], ancient city of Pannonia. The site is near modern Sremska Mitrovica, NW Serbia. Sirmium was unimportant until occupied late in the 1st cent. B.C. by the Romans in the conquest of Pannonia. It was prominent later, especially in the 3d and 4th cent. A.D., and became the chief city of Lower Pannonia.
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