[vkazanov]

Just a note: there was no such state as "Russia" back then. The whole region was a number a big and small duchies left after the fall of the older feudal state, Kievan Rus'.

Later, by the end of the 15-th century, most of them were reunited under the rule of either the Grand Duchy of Lithuania or the Grand Duchy of Vladimir and Moscow (which would, two centuries later, become what we know today as Russian Empire, or Russia).

Anyway, Mongols had no problems crashing both quite a few times between 13-16 centuries. :-) Every state there was in the region back then had to paid the Horde.