The Mongols were close to unstoppable, but it’s an interesting question what might have happened had they penetrated further west into areas of Europe that weren’t natural horse country.
In fact, we got a preview of that answer when they tried to extend into India. Basically they ended up saying ... "Thanks but no thanks!"
Genghis Khan's first raid was supposedly to rescue is wife, who had been kidnapped by another tribe. I got a sense that after that he just found he had to stay on the offensive until it became a way of life.
The Mongols were also the first true globalists.
I often wonder if those cities that simply let the Mongols take over didn't end up better off than they were before the Mongols, because they no longer had a need for big defensive forces. (The tributes those cities paid may have been less than the taxes we pay here in the US today!)