 Complete the following phrase:  Kallinikos development of Greek fire came at a critical moment in the Byzantine Empires history: weakened by its long wars with Sassanid Persia, the Byzantines had been unable to effectively resist the onslaught of the Muslim conquests. Within a generation, Syria, Palestine, and Egypt had fallen to the Arabs, who in set out to conquer the imperial capital of Constantinople. Greek fire was used to great effect against the Muslim fleets, helping to repel the Muslims at the first and second Arab sieges of the city. Records of its use in later naval battles against the Saracens are more sporadic, but it did secure a number of victories, especially in the phase of Byzantine expansion in the late 9th and early 10th centuries. Utilisation of the substance was prominent in Byzantine civil wars, chiefly the revolt of the thematic fleets in 727 and the large-scale rebellion led by Thomas the Slav in 821823. In both cases, the rebel fleets were defeated by the Constantinople-based central Imperial Fleet through the use of Greek fire. The Byzantines also used the weapon to devastating effect against the various Rus raids on the Bosporus, especially those of 