On the other hand, the historian M. A. Shaban considers the campaign a "limited success" at best, and criticizes Harun's "single-minded" attention to the Byzantines as a "totally misguided effort". According to Shaban, not only did the Byzantines have no real ability (or intention) to seriously threaten the Caliphate, but Harun's recruitment drive led to the influx of eastern soldiers from Khurasan, which antagonized the traditional Syrian–Iraqi military elites and created rifts that contributed to the Fourth Fitna, the Abbasid civil war that broke out after Harun's death. This conflict, between Harun's sons al-Amin () and al-Ma'mun (), meant that the Abbasid Caliphate was not able to exploit the Byzantine reversals in the Balkans. Indeed, the 806 campaign and the ineffectual raids of 807 mark the last major, centrally organized, Abbasid expeditions against Byzantium for over twenty years. Isolated raids and counter-raids continued at land as well as at sea, and, independently of the Abbasids, local Muslim leaders conquered Crete and launched the conquest of Sicily in the 820s. Nevertheless, large-scale operations over the land border in eastern Asia Minor between the two empires resumed only after the accession of Emperor Theophilos (), whose confrontations with the caliphs al-Ma'mun and al-Mu'tasim () culminated in the great invasions by al-Ma'mun in 830–833, and the Sack of Amorium by al-Mu'tasim in 838.