early in 627, the göktürks and their khazar allies approached the caspian gates at derbent. this newly built stronghold was the only gate to the fertile land of aghvania (modern-day azerbaijan). lev gumilev observes that the lightly armed militia of aghvania was no match against the hordes of heavy cavalry led by tong yabghu. his troops stormed derbent and swarmed over aghvania, plundering it thoroughly. the fall and sack of derbent was described in detail by the armenian historian movses kagankatvatsi, thought to have been an eyewitness to the event: