The following year the murder of the Caliph brought on a civil war among the Saracens, in consequence of which Muaviah arranged a truce with Constantine. The latter was thus enabled to turn his attention to the beating back of the Slavs in the east and the recovery of imperial possessions in the west, notably the city and province of Carthage. During the last of these campaigns he was killed by a slave. The death of this energetic and able ruler seemed to Muaviah the opportunity to begin fresh operations against the Christian empire. Three great armies invaded the territory of the Cross. One plundered Syracuse, another seized and fortified a post that threatened the existence of Carthage, a third pushed to the shores of the Sea of Marmora. These were, however, only preliminary to the grand assault on the capital itself. In 673 a great Arab armada forced the Hellespont and captured Cyzicus. With this as a base, the fleet landed an army on the northern shore of the Sea of Marmora. By these means Constantinople was invested by land and sea. But the great walls proved impregnable against the attacks of the army, and the Christian fleet, sheltered in the Golden Horn, was able to sally out from time to time and make successful raids on detachments of the Saracen ships. This state of affairs continued for six months, after which Muaviah retired with his army to Cyzicus, leaving a strong naval guard to hold the straits. The next spring Muaviah again landed his army on the European side and besieged the city for several months. The second year's operations were no more successful than the first, and again the Arab force retired to Cyzicus for the winter. The Arab commander was determined to stick it out until he had forced the surrender of the city by sheer exhaustion, but his plan had a fatal error. During the winter months the land blockade was abandoned, with the result that supplies for the next year's siege were readily collected for the beleaguered city. Emperor and citizens alike rose to the emergency with a spirit of devotion that burned brighter with every year of the siege. Meanwhile the Christians of the outlying provinces of Syria and Africa were also fighting stubbornly and with considerable success against the enemy. The year 676 passed without any material change in the situation. [Illustration: CONSTANTINOPLE AND VICINITY]