Why did a prince of Kyiv choose Orthodox Christianity in 988?

Most likely for reasons of politics, even though the famous story tells it as a matter of beauty. According to the Primary Chronicle, Prince Vladimir of Kyiv chose the Christianity of Constantinople after his envoys were dazzled by its churches. Historians think it is more likely that he adopted Byzantine Christianity to strengthen his diplomatic relations with Constantinople, and the strongest hint of a deal is the marriage he demanded from the emperor.

The legend goes like this. Vladimir decided to leave traditional Slavic paganism and sent advisors and warriors out to look at other faiths. They rejected Islam, partly because it prohibited alcohol, and Judaism, because the god of the Jews had let his chosen people lose their country. They found the ceremonies of the Roman church dull. But in Constantinople they were so astounded by the cathedral of Hagia Sophia and its service that they made up their minds on the spot.

The real events look more like a bargain. In 987 a rebel named Bardas Phokas proclaimed himself emperor and rose up against Basil II, who appealed to Kyiv for support. Vladimir agreed, but demanded to marry the emperor's sister Anne in exchange. In spring 988 the Rus' army helped defeat Phokas, but Basil II then hesitated over the marriage. So Vladimir attacked and pillaged Chersonesus, a message that Constantinople could be next. The emperor gave in.

Back in Kyiv, Vladimir ordered the pagan idols destroyed, including the pantheon he had himself built only eight years before, and the city's inhabitants were baptized in the Dnieper. Residents were told to come to the river or risk becoming the prince's enemies. The year 988 is the traditional date, though it is disputed, and in places like Novgorod the change did not come easily: people there were baptized only in 989, after fierce clashes.