The Sack of Thessalonica in 904 by Saracen pirates was one of the worst disasters to befall the Byzantine Empire in the 10th century. A Muslim fleet, led by the renegade Leo of Tripoli, and with the imperial capital of Constantinople as its initial target, sailed from Syria. The Muslims were deterred from attacking Constantinople, and instead turned to Thessalonica, totally surprising the Byzantines, whose navy was unable to react in time. The city walls, especially towards the sea, were in disrepair, while the city's two commanders issued conflicting orders.

• http://en.wikipedia.org/wiki/Sack of Thessalonica (904)