Complex sentence: The first ban who swore allegiance to a king, (Emeric of Hungary) was Ban Kulin in 1203, but during the 13th century the supremacy of the Kings of Hungary over Bosnia was only theoretical.
Split into simpler sentences: The first ban who swore allegiance to a king, (Emeric of Hungary) was Ban Kulin in 1203. However, during the 13th century, the supremacy of the Kings of Hungary over Bosnia was only theoretical.