For the Arabs all went well at first. Unopposed they transported a part of their army to the European shore, moved toward Constantinople and invested it by land and sea. One detachment was sent to cover Adrianople, which was occupied by a Christian garrison; the rest of the force concentrated on the capital itself. Meanwhile the Christian fleet lay anchored in the shelter of the Golden Horn, protected by a boom of chains and logs. As the Saracen ships came up to occupy the straits above the city they fell into confusion in trying to stem the rapid current. Seeing his opportunity, the emperor ordered the boom opened, and leading the way in his flagship, he fell upon the huddle of Saracen vessels in the channel. The latter could make little resistance, and before the main body of the fleet could work up to the rescue, the Christians had destroyed twenty and taken a number of prizes back to the Horn. Again Greek fire had proved its deadly efficacy. Elated with this success, Leo ordered the boom opened wide and, lying in battle order at the mouth of the Horn, he challenged the Arab fleet to attack. But such was the terror inspired by Greek fire that the Grand Vizier, in spite of his enormous superiority in numbers, declined to close. Instead he withdrew his dromons out of the Bosphorus and thereafter followed the less risky policy of a blockade. This initial success of the Christian fleet had the important effect of leaving open the sea route to the Black Sea, through which supplies could still reach the beleaguered city. The Arabs then sat down to wear out the defenders by a protracted siege on land and sea. In the spring of 718 the new army and the two new fleets arrived on the scene. One of the latter succeeded, probably by night, in passing through the Bosphorus and closing the last inlet to the city. The situation for the defenders became desperate. Many of the men serving on these new fleets, however, were Christians. These took every opportunity to desert, and gave important information to the emperor as to the disposition of the Arab ships. Acting on this knowledge, Leo took his fleet out from the shelter of the boom and moved up the straits against the African and Egyptian squadrons that were blockading the northern exit. The deserters guided him to where these squadrons lay, at anchor and unprepared for action. What followed was a massacre rather than a battle. The Christian members of the crews deserted wholesale and turned upon their Moslem officers. Ship after ship was rammed by the Christian dromons or set on fire by the terrible substance which every Arab regarded with superstitious dread. Some were driven ashore, others captured, many more sunk or burnt to the water's edge. Of a total of nearly 800 vessels practically nothing was left.