 At the same time, the need to protect the Anatolian heartland of Byzantium from the Arab raids led to the creation, in the later 8th and early 9th centuries, of a series of small frontier districts, the "kleisourai" or "kleisourarchiai" ("defiles, enclosures").The term was previously used to signify strategically important, fortified mountain passages, and was now expanded to entire districts which formed separate commands under a "kleisourarchēs", tasked with guerrilla warfare and locally countering small to mid-scale incursions and raids.Gradually, most of these were elevated to full themes.