In the 11th century, Sirmium was a residence of Sermon, a duke of Syrmia, who was a vassal of the Bulgarian Samuil. After 1018, the city was again included into the Byzantine Empire, and since the end of the 11th century, Sirmium was a subject of a dispute between the Byzantine Empire and the Kingdom of Hungary, until 1180 when the Byzantine Empire gave up Sirmium, surrendering it to the Kingdom of Hungary. In the 11th century, a Byzantine province named Theme of Sirmium had its capital in this city.