In the mid 7th century AD, following the Muslim conquest of Persia, Islam penetrated into areas that would later become part of European Russia. A centuries later example that can be counted amongst the earliest introductions of Islam into Eastern Europe came about through the work of an early 11th-century Muslim prisoner whom the Byzantines captured during one of their wars against Muslims. The Muslim prisoner was brought into the territory of the Pechenegs, where he taught and converted individuals to Islam. Little is known about the timeline of the Islamization of Inner Asia and of the Turkic peoples who lay beyond the bounds of the caliphate. Around the 7th and 8th centuries some states of Turkic peoples existed - like the Turkic Khazar Khaganate (see "Khazar-Arab Wars") and the Turkic Turgesh Khaganate, which fought against the caliphate in order to stop Arabization and Islamization in Asia. From the 9th century onwards, the Turks (at least individually, if not yet through adoption by their states) began to convert to Islam. Histories merely note the fact of pre-Mongol Central Asia's Islamization. The Bulgars of the Volga (to whom the modern Volga Tatars trace their Islamic roots) adopted Islam by the 10th century. under Almış. When the Franciscan friar William of Rubruck visited the encampment of Batu Khan of the Golden Horde, who had recently (in the 1240s) completed the Mongol invasion of Volga Bulgaria, he noted "I wonder what devil carried the law of Machomet there".