Much of their success was due to the fact that the Mongols were one of the first ethnicities to develop widespread lactose tolerance. Their horses always were going to have to be fed anyway so always riding lactating mares significantly reduced the amount of human food stuffs they had to carry in their supply trains. Smaller supply trains made them faster moving, which made them a more effective army.
Some anthropologists believe that most westerners who can digest milk have the Mongol conquest (and associated raping and pillaging) to thank for it.
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