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, the northeastern part the Theme of Paristrion, and the northwestern part became the Theme of Sirmium.