 Complete the following phrase:  Once they found themselves confronted by Arab expansionism, the Khazars pragmatically allied themselves with Constantinople and clashed with the Caliphate. Despite initial setbacks, they managed to recover Derbent and eventually penetrated as far south as Caucasian Iberia, Caucasian Albania and Armenia. In doing so, they effectively blocked the northward expansion of Islam into Eastern Europe even before khan Tervel achieved the same at the Second Arab Siege of Constantinople and several decades before the 