Trying to answer this question for myself by doing a bit of research, I have no sense of how a few tribes in Mongolia united and were able to rapidly obliterate multiple huge, well-established empires. For example, the conquests of European empires during the colonialism-era can be attributed to technological and biological advantages (e.g., "guns, germs and steel"). Or, Napoleon's sudden advance over most of Europe can be attributed to the advantages of professional officers and universal conscription over militaries limited by the obsolete conventions of an earlier time. But, what did the Mongolians have that allowed them to dominate from Vienna to Japan, from Siberia to Java?
hi! more contributions are welcome, but meawhile, you may be interested in responses to some similar questions. (I did a quick search for "how mongol" and "mongol conquer", so, readers, if I've missed some particularly insightful posts, do reply here with links)
Why were the mongols so hard to beat? and follow-on What made the mongols so powerful?
How did the Mongol Empire capture and hold such a large area?
How did the Mongols overpower a lot of great civilizations, with just the horseback archer?
If the Mongols were nomadic people, how did they maintain control of land that they conquered?
Or, Napoleon's sudden advance over most of Europe can be attributed to the advantages of professional officers and universal conscription over militaries limited by the obsolete conventions of an earlier time.
From what I've read the Mongols had something vaguely analogous to this, the great hunt was apparently a thing Mongol tribes did yearly where large amounts of horsebacked men would drive wild animals to a central place where they could then be killed. Apparently the nature of this doing required impressive coordination including making large amounts of men react to conditions, coordination that when put on the battlefield was a big advantage at the time allowing the Mongols to outreact their enemies
Also all the advantages everyone else is talking about that you get from being a horse riding nomad society
Source: I want to say it was The Mongols by David Morgan that describes the great hunt and its implications but it might have been The Mongols by WB Bartlett that did it (I have neither book on me at the moment)
It is common to see the Mongols as barbarians or savages, but one should remember that before the modern era in history, steppe nomadic groups such as - but definitely not limited to - the Mongols were major threats to the more sedentary, "advanced" civilizations of Eurasia. The Mongols are only one example in a long line of steppe nomadic peoples (the sort of people who excel at cavalry warfare) that managed to at least temporarily united and take on groups of people who we see as more civilized. Think of the Parthians who managed to take over Persia, the Huns who almost toppled the Western Roman Empire, the Turks who set up the vast Seljuk and Ottoman Empires, or the Manchus who conquered China and set up its last dynasty. There are other people here that can probably answer better than I can what specifically the Mongols brought to the table that made them that effective, but my point is really to say that the Mongols aren't quite the great historical exception that they are sometimes made out to be.
Something a lot of people don't realize is that while mongols weren't terribly numerous, they were all fighters. The people the mongols conquered had soldiers but the military was only one aspect of their societies. For the mongols on the other hand the military was the first and foremost part of their society. Because the tribes were constantly at war with one another before unification, every mongol man was a soldier along with whatever else he did.