Constans's son Constantine IV (668–685) had been made co-emperor in 654, and ruled in the east while his father campaigned in the west (662–668), succeeding him on the latter's death. Almost immediately he had to deal with Arab attacks on Amorium in Phrygia in 646 and Chalcedon in Bithynia. This was followed by the capture of Cyzicus in Mysia 670, as well as Smyrna and other coastal cities, finally attacking Constantinople itself in 674. This First Siege of Constantinople demonstrated how vulnerable the city was to attack, but also its strengths which ultimately prevailed, the Arabs lifting the siege in 678, and after further setbacks signing another truce which allowed Constantine to concentrate on the Balkan threat. For a long time the Danube in the Balkans had been considered the frontier that must be defended to maintain the integrity of Thrace. Now a new Slavic threat, the Bulgars crossed the Danube and inflicted heavy losses on the Byzantines in 681. Faced with the uncompromising religious controversy that had perplexed his father he convened another ecumenical council, the sixth (Third Council of Constantinople) in 680, which condemned Monotheletism. He also initiated a series of civil and military reform to cope with the shrunken and threatened empire. This was to do away with the original system of provinces with a new administrative structure based on "themata" (themes), the remaining parts of Anatolia being divided amongst seven "themata". When he died in 668 he was succeeded by his son, Justinian II (685–695, 705–711), co-emperor since 681.