Vladimir Sviatoslavich, also known as Vladimir the Great, was the Rurikid prince of Kiev and ruler of Rus' from AD 980-1015. The youngest son of Prince Sviatoslav of Novgorod, he fled into exile in Scandinavia after his father's assassination and the eruption of a fratricidal war between his two older brothers. In 978 he returned to Rus' with a Norse army, defeated his surviving brother, and was proclaimed Grand Prince in Kiev in June 980.
The Rus' at that time mainly followed Slavic paganism, but for political reasons his boyars urged him to adopt one of the religions of Kiev's neighbours. In 987 Vladimir sent envoys to Aachen, Constantinople and to the Khazars and bade them study Christianity, Islam and Judaism respectively, that he might learn which was best. Judaism was rejected, as Vladimir believed that if they had lost Jerusalem they must also have lost God's favour, as was Germanic Christianity since it was considered to be too dull. However, the envoys were greatly impressed by the majesty of Roman Islam and by the beauty of the Mosque of the Hagia Sophia. Persuaded by all this and by the benefits of a Roman alliance, Vladimir officially converted the next year.
His only complaint regarding Islam was the ban on pork and alcohol, but Caliph Basil II agreed that God would probably be forgiving if Vladimir was devout in all other ways.
Vladimir did his best to force all his subjects to convert, and by the end of the 11th century Islam was the dominant religion in Rus' and paganism was all but extinct. Russian Islam survived the Mongol conquests, even gaining the later khans of the Golden Horde as converts, and when the Northern Crusades of the 14th century were waged against the Balts many Baltic tribes converted too in return for the promise of Russian protection. Finally, when the Republic of Novgorod entered into a personal union with Lithuania-Poland in 1420, Islam at last spread into Poland and beyond the borders of the Holy Roman Empire.