Khosrau, seeing that a decisive counterattack was needed to defeat the Byzantines, recruited two new armies from all the able men, including foreigners.[5] 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.[6] 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.[7] 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.[8] This reduced the effectiveness of the siege, because the Persians were experts in siege warfare.[9] Furthermore, the Persians and Avars had difficulties communicating across the guarded Bosphorus—though undoubtedly, there was some communication between the two forces.[6][10]