The Armenian writer Sebeos records in detail a great Arab attack on the Byzantine capital in 654, which ended in disastrous failure. No parallel accounts are known, and Sebeos’ report has not been seriously considered. Yet Sebeos, otherwise known to be a reliable author, was writing only shortly after the supposed event. The event is plausible in its historical context. Allusions to it in several historical sources may be remnants of written records, parallel to Sebeos’ account, which disappeared after the condemnation of Monotbeletism in 680–1. Indirect evidence of the attack from several other Christian sources and, to a lesser degree, from the Islamic tradition also tends to confirm Sebeos’ report.