My dad said that the reason the mongol empire grew so large was because there was no real challenge to oppose them, though I agree with him in certain cases, I totally disagree with him on this one. Can anyone supply me with the great challenges that the Mongols faced during their expansion, or maybe I'm wrong...
Thanks in advance!
For the 1211 campaign, Jin gathered together 400500k to fight Genghis Khan's 90110k. Especially notable that at Badger's Mouth the Mongols had to fight dismounted due to the terrain, but they still won.
Similarly the Khwarezmian Empire also deployed about 400k men against the Mongols, who even counting auxiliaries had 150~250k men.