Abd ar-Rahman before they proceeded to the Hellespont, into which they sailed in about April 674. From April to September 674 the fleet lay moored from the promontory of Hebdomon, on the Propontis, as far as the promontory of Kyklobion, near the Golden Gate, and throughout those months continued to engage with the Byzantine fleet which defended the harbour from morning to evening. Knowing that it was only a matter of time before Constantinople was under siege, Constantine had ensured that the city was well provisioned. He also constructed a large number of fireships and fast-sailing boats provided with tubes or siphons for squirting fire. This is the first known use of Greek fire in combat, which was one of the key advantages that the Byzantines possessed. In September, the Arabs, having failed in their attempts to take the city, sailed to Cyzicus, which they made their winter quarters. Over the following five years, the Arabs returned each spring to continue the siege of Constantinople, but with the same results. The city survived, and finally in 678 the Arabs were forced to raise the siege. The Arabs withdrew and were almost simultaneously defeated on land in Lycia in Anatolia. This unexpected reverse forced Muawiyah I to seek a truce with Constantine. The terms of the concluded truce required the Arabs to evacuate the islands they had seized in the Aegean, and for the Byzantines to pay an annual tribute to the Caliphate consisting of fifty slaves, fifty horses, and 300,000 nomismata. The raising of the siege allowed Constantine to go to the relief of Thessalonica, still under siege from the Sclaveni. Later reign With the temporary passing of the Arab threat, Constantine turned his attention to the Church, which was torn between Monothelitism and Orthodoxy. In November 680 Constantine convened the Sixth Ecumenical Council (also known as the Third Council of Constantinople). Constantine presided in person during the formal aspects of the proceedings (the first eleven sittings and then the eighteenth), surrounded by his court officials, but he took no active role in the theological discussions. The Council reaffirmed the Orthodox doctrines of the Council of Chalcedon in 451. This solved the controversy over monothelitism; conveniently for the Empire, most monothelites were now under the control of the Umayyad Caliphate. The council closed in September 681. Due to the ongoing conflicts with the Arabs during the 670s, Constantine had been