With Kiev weakened and central authority in disarray, suddenly a terrible disaster struck, an event so devastating that some Russians think its effects are still felt today. Genghis Khan had established a powerful Mongol Empire far to the east, and after his death in 1227, his descendants continued his efforts to expand control over much of Asia and on into Europe. The Mongols swept down out of Asia in 1237 and destroyed many Russian towns, burning and killing as they went. Kiev fell in 1240, effectively ending the Kievan age in Russian history. The Mongols initially were able to take control of all Russia except the Novgorod region, but even this area was later forced to submit to Mongol authority.
The Mongols formed a state called the Golden Horde; its center was at Sarai, northwest of the Caspian Sea. The Russian rulers, the princes of the various regions, had to travel there to pledge their loyalty. The Mongols were mostly interested in collecting tribute from the Russians and in getting fresh troops for their armies, rather than in imposing their culture on the Russians. Yet their emphasis on a very strong central authority placed its stamp on Russia and became a feature of Russian life that endured long after the Mongol rule ended. Mongol rule also increased Russia's isolation from western Europe and contributed to the sense that Russians have of themselves as a combination of east and west. The subjugation of the Russian people by the Mongol Empire became known as the Tartar (or Tatar) Yoke, after one of the Mongol groups. (Present day Tatars of Tatarstan are NOT Mongols but descendants of Bulgar Turks - the Term "Tatar" has several meanings) Today Russians have a saying, "Scratch a Russian and you'll find a Tartar." They also say, "An uninvited guest is worse than a Tartar." (whatever that means??)
from Cultures and Customs of Russia
by Sydney Shultze 2000