Excerpts from Books and Wikipedia
"In 987 the Byzantine emperor, Basil II, was forced to ask Vladimir for help against usurpers who threatened the imperial throne. Vladimir agreed on condition that he be allowed to marry the emperor's sister, Anne. To strengthen his bargaining position, Vladimir took an army into the area now known as Sebastopol and threatened to attack Constantinople. Basil agreed, providing Vladimir was baptized. So, in order to secure an advantageous marriage, Vladimir was baptized in 988. Vladimir proceeded to order the tearing down of pagan temples, the destruction of idols, and the baptism of his people in the Greek Byzantine form of Christianity." [DK Timelines, p. 152]