Here is a very informative LINK that shows the spread of the Mongol empire. Under Genghis Khan and his successors it became the largest contiguous empire in history. Notice that at its apex in 1279 it was even moving into Europe.
The methods of the Mongols in conquered territories were notorious brutal, sometimes involving wholesale massacres of the civilian populations. Given the limitations of travel and communication in those days, imagine the abject terror that must have come upon the people in the path of the Mongol armies (which, of course, was the specific object of their methods).
For those of you who have enjoyed the cantata Alexander Nevsky by Prokofiev, you will recall that the first movement of that piece (and the first section of the movie for which he originally wrote the music) was called "Russia Under the Mongol Yoke." A significant part of Nevsky's political career involved his relations with the Mongols. He apparently felt that paying tribute to the Khans would relieve any pressure of repeated Mongol invasions and that it was money well spent.
How could anyone govern an empire stretching from the Pacific Ocean to the Baltic Sea and from southern Siberia to India? That ought to make an interesting study.