In the 6th century, another Byzantine province existed in this area. It was known as
Pannonia and also had its capital in
Sirmium, but was much smaller in size. In the beginning of the 11th century, the area which later became the Theme of Sirmium lay within the borders of the
First Bulgarian Empire, under Tsar
Samuil and the local duke (voivode) known as
Sermon ruled over Sirmium and surrounding area. In a long war, the Byzantine emperor
Basil II conquered Bulgaria, and established new Byzantine themes and other local governorates under generals (
strategoi) on its territory. The central part of Samuil's realm became the
Theme of Bulgaria and the northwestern part became the Theme of Sirmium. The bishoprics of these lands, including the bishopric of Sirmium, are mentioned as the westernmost eparchies of the
Archbishopric of Ohrid in 1019–20. ==Geography==