First, the ruler of Kiev was called the Grand Prince, and he became the model for future Russian kings. Also, the early Grand Princes made a fateful decision. They became Byzantine Christians. 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.