Several years later, in 626, he captured Lazistan (Colchis). Later that same year, Shahrbaraz advanced on Chalcedon on the Bosphoros and attempted to capture Constantinople with the help of Iran's Avar and Slavic allies. In this siege of Constantinople in 626, the combined Sassanid, Slavic and Avar forces failed to capture the Byzantine capital city. The Avars did not have the patience or technology to conquer the city. On top of that, the Iranians, who were siege warfare experts, were unable to transport their troops and equipment to the other side of the Bosphorus where their Slavic and Avar allies were located, due to heavy guarding of the strait by the Byzantine navy. Furthermore, the walls of Constantinople easily defended against the siege towers and engines. Another reason was that the Persians and Slavs did not have a strong enough navy to ignore the sea walls and establish a channel of communication. The lack of supplies for the Avars eventually caused them to abandon the siege. As this maneuver failed, his forces were defeated, and he withdrew his army from Anatolia later in 628.