The Arab armies moved on up the western coast of the Caspian Sea to the town the Arabs called Bab al-Abwab, the Gate of Gates, which is now called Derbent. It was here that the main range of the Caucasus mountains came down to the sea coast. At this point the Sasanians had established a fortified outpost. The long, strong stone walls still run from the sea to the spur of the mountains. This was frontier territory. Beyond the wall was nomad country, the vast plains of what is now southern Russia.

The commander of the Sasanian garrison was one Shahrbaraz. He was very conscious of his aristocratic origins, and clearly had little sympathy with the people of the Caucasus and the Armenians who surrounded him. Knowing that the Sasanian regime elsewhere had collapsed, he sought instead to make common cause with the Arab leaders, entering into a series of negotiations in which it was agreed that he and his men should be exempt from paying a poll tax in exchange for military service in the frontier army. In this way the remaining elements of the Sasanian army were not defeated, but incorporated into the armies of Islam. No doubt some of them soon came to convert to Islam, too.

Interestingly, while the Arab commanders were keen to attack the nomads beyond the wall at Bab, the experienced Persians warned against it, saying effectively that they should let sleeping dogs lie. The Arabs did launch raids north of the wall, but no permanent gains were made. In the long run, the frontier established at the wall in 641-2 has remained the frontier of the Muslim world in the eastern Caucasus to the present day.

From Hugh Kennedy’s The Great Arab Conquests: How the Spread of Islam Changed the World We Live In, London: Weidenfeld & Nicholson, 2007, p. 180.