Marwan now launched a two-pronged offensive against the Khazars: 30,000 men (including most of the levies from the Caucasian principalities) under the governor of Derbent, Asid ibn Zafir al-Sulami, advanced north along the coast of the Caspian Sea, while Marwan himself with the bulk of his forces crossed the Darial Pass. The invasion met little resistance; the Arab sources report that Marwan had deliberately detained the Khazar envoy and only let him go with a declaration of war once he was deep in Khazar territory. The two Arab armies converged on Samandar, where a great review was held; according to Ibn A'tham, the troops were issued new clothing in white—the dynastic colour of the Umayyads—as well as new spears. From there Marwan pushed on, reaching, according to some Arab sources, the Khazar capital of al-Bayda on the Volga. The "khagan" withdrew towards the Ural Mountains, but left behind a considerable force to protect the capital. This was a "spectacularly deep penetration", according to Blankinship, but of little strategic value: the 10th-century travellers Ibn Fadlan and Istakhri describe the Khazar capital in their time as little more than a large encampment, and there is no evidence that it had been larger or more urbanized in the past.