The Qağanate of the Khazars thus took shape out of the ruins of this nomadic empire as it broke up under pressure from the Tang dynasty armies to the east sometime between 630–650.  After their conquest of the lower Volga region to the East and an area westwards between the Danube and the Dniepr , and their subjugation of the Onoğur - Bulğar union, sometime around 670, a properly constituted Khazar Qağanate emerges,  becoming the westernmost successor state of the formidable Göktürk Qağanate after its disintegration. According to Omeljan Pritsak , the language of the Onoğur-Bulğar federation was to become the lingua franca of Khazaria  as it developed into what Lev Gumilev called a 'steppe Atlantis' ( stepnaja Atlantida / Степная Атлантида).  Historians have often referred to this period of Khazar domination as the Pax Khazarica since the state became an international trading hub permitting Western Eurasian merchants safe transit across it to pursue their business without interference.  The high status soon to be accorded this empire to the north is attested by Ibn al-Balḫî 's Fârsnâma (c. 1100), which relates that the Sasanian Shah, Ḫusraw 1, Anûsîrvân , placed three thrones by his own, one for the King of China, a second for the King of Byzantium, and a third for the king of the Khazars. Though anachronistic in retrodating the Khazars to this period, the legend, in placing the Khazar qağan on a throne with equal status to kings of the other two superpowers, bears witness to the reputation won by the Khazars from early times.