Why were the Mongol-Tatars able to seize Russia?

The Mongol-Tatars captured Russia and held it as their ulus for more than two hundred years. There are many reasons why the nomads managed to subjugate Russia in such a short time. In Russia during this period there was fragmentation. The princes were in conflict and tried in every possible way to isolate themselves from each other and strengthen their principality. It was at this moment that Batu went to the state. The khan one by one defeated the princes who had never wanted to unite. There was no strong centralized power in Russia, so the state failed to resist the large army of Mongols.