In the summer of 904, a Byzantine renegade in Abbasid service, Leo of Tripoli, led a major naval expedition of 54 vessels from the Syrian and Egyptian fleets, whose initial target reportedly was Constantinople itself. The Arab fleet penetrated the Dardanelles and sacked Abydos, as the Byzantine navy under the "droungarios" Eustathios Argyros was reluctant to confront them. Emperor Leo replaced Argyros with the more energetic Himerios, but Leo of Tripoli forestalled the Byzantines, turning back west and heading for the Empire's second city, Thessalonica, which he sacked after a three-day siege on 31 July 904. The sack of the city brought the Muslim fleet enormous booty and many captives who were taken to be sold as slaves, including the eyewitness John Kaminiates, who wrote the main account of the city's siege and fall.