The reasons for the success of the Mongols?

The Mongols in military terms had advantages in the number of troops and resources that fed it, because they already managed to conquer many peoples before the attack on Russia, which imposed tribute. From the same place came new replenishment of soldiers. At the same time, Russia was fragmented and could not unite together to repel the attack of the enemy, which previously always helped. Many of the princes were pleased with the fact that they were able to rule under the rule of the Mongols – it was enough that they paid tribute from the places of their rule.