According to legend, Prince Vladimir chose to convert the Rus to Byzantine Christianity in the 11th century. He purportedly chose Christianity over Islam because of Islam's prohibition on alcohol, saying, Drink is the joy of the Russian. Anyway, the Kievian Rus eventually fell in 1240, when these guys showed up and replaced them. After that, they'd been at war with pastoral nomads for centuries, from the Khazars to the Pechenegs to the Cumans, and they were tired. Which made them easy targets. The period of Mongol rule over Russia is also known as Apennage Russia. And Apennage is a princedom, and this period basically featured a bunch of Russian princes vying for control over territory, which is not a recipe for political stability or economic growth, another theme that will re-emerge in Russian history.