Khosrau, seeing that a decisive counterattack was needed to defeat the Byzantines, recruited two new armies from all the able men, including foreigners.
Shahin was entrusted with 50,000 men and stayed in Mesopotamia and
Armenia to prevent Heraclius from invading Persia; a smaller army under
Shahrbaraz slipped through Heraclius' flanks and bee-lined for
Chalcedon across the
Bosphorus from Constantinople. Khosrau also coordinated with the Khagan of the
Avars so as to launch a coordinated attack on Constantinople from both European and Asiatic sides. The Persian army stationed themselves at Chalcedon, while the Avars placed themselves on the European side of Constantinople and probably destroyed the
Aqueduct of Valens. Because of the
Byzantine navy's control of the Bosphorus strait, however, the Persians could not send troops to the European side to aid their ally. This reduced the effectiveness of the siege, because the Persians were formidable in
siege warfare. Furthermore, the Persians and Avars had difficulties communicating across the guarded Bosphorus—though undoubtedly, there was some communication between the two forces. The defense of Constantinople was under the command of Patriarch
Sergius and the patrician
Bonus. Upon hearing the news, Heraclius split his army into three parts; although he judged that the capital was relatively safe, he still sent some reinforcements to Constantinople to boost the morale of the defenders. Another part of the army was under the command of his brother Theodore and was sent to deal with Shahin, while the third and smallest part would remain under his own control, intending to raid the Persian heartland. On 29 June 626, a coordinated assault on the walls began. Inside the walls, some 12,000 well-trained Byzantine cavalry troops (presumably dismounted) defended the city against the forces of some 80,000
Avars and
Sclaveni who were determined to remove all Roman imperial rule over Europe. However, it was only when the Avars began moving forward heavy siege equipment towards the
Theodosian Walls that their intention to lay a siege became clear. Despite continuous bombardment for a month, morale was high inside the walls of Constantinople because of
Patriarch Sergius' religious fervor and his processions along the wall with an icon, which could be the
icon of the
Virgin Mary, inspiring the belief that the Byzantines were under divine protection. Furthermore, the patriarch's cries for religious zeal among the peasantry around Constantinople was made ever more effective by the fact that they were facing
heathens. Consequently, every assault became a doomed effort. When the Avar-Slavic fleet and the Persian fleet were sunk in two different naval engagements, the attackers panicked and fled, abandoning the siege, apparently under the belief that
divine intervention had won the day for Byzantium. On 7 August, a fleet of Persian rafts ferrying troops across the Bosphorus were surrounded and destroyed by Byzantine ships. The Slavs under the Avars attempted to attack the sea walls from across the
Golden Horn, while the main Avar host attacked the land walls. Patrician Bonus' galleys rammed and destroyed the Slavic boats; the Avar land assault from 6 August to the 7th also failed. With the news that Theodore had decisively triumphed over Shahin (supposedly leading Shahin to die from depression), the Avars retreated to the Balkan hinterland within two days, never to seriously threaten Constantinople again. Even though the army of
Shahrbaraz was still encamped at Chalcedon, the threat to Constantinople was over. In thanks for the lifting of the siege and the supposed divine protection of the Virgin Mary, a new proemium for the celebrated
Akathist Hymn was written by an unknown author, possibly Patriarch Sergius or
George of Pisidia. ==Aftermath==