A fleet was based in Constantinople at least since the 7th century, and indeed played a central role in the repulsion of the two Arab sieges of Constantinople in 674–678 and 717–718, but the exact date of the establishment of the Imperial Fleet (βασιλικὸς στόλος, "basilikos stolos", or βασιλικὸν πλόϊμον, "basilikon ploïmon") as a distinct command is unclear. The Irish historian J. B. Bury, followed by the French Byzaninist Rodolphe Guilland, considered it "not improbable" that the Imperial Fleet existed as a subordinate command under the "stratēgos tōn karabisianōn" already in the 7th century. Certainly the "droungarios" of the Fleet first appears in the "Taktikon Uspensky" of ; and as there is little evidence for major fleets operating from Constantinople during the 8th century, the Greek Byzantinist Hélène Ahrweiler dated the fleet's creation to the early 9th century. From that point on, the Imperial Fleet formed the main naval reserve force and provided the core of various expeditionary fleets.