It took Ghengis Khan- one of if not the most intelligent and capable political and military leaders that humanity has ever birthed- decades of effort to conquer only part of China.
Decades of effort to conquer only part of China?
On the one hand this statement is true and on the other hand it is wrong.
TRUE:
- there were a few encounters/campaigns of Ghengis Khan and his generals with the North China (Jin dynasty) and that's true - there were decades between the first and the last encounter.
WRONG:
- that was
not one exhausting uninterrupted war which lasted decades and decades and decades and decades. It was
not like every year tens of thousands of the Mongols went to China to keep the conquest going on a permanent regular basis.
A few minor encounters aside there were only several major Mongol campaigns in China, some of them surprisingly short and sweeping. After every campaign most of the (ethnic) Mongols withdrew from China, only some of the Mongols were left (literally a thousand or so), who controlled the territory mostly by the fear that the main Mongol forces would come back and punish those who misbehaved.
During these periods the non-Mongol troops were extensively used in China, who were no match for the "true" Mongol forces, barely enough to keep what was already conquered.
So, the
actual conquering of the North China (when the invading Mongols were in tens of thousands, not in hundreds) took 3-4 years at best (and that is counting the major campaigns headed not only by Ghengis Khan himself, but by his generals).
You see, Ghengis Khan had his hands full with other wars of his. Some of them are poorly documented (like the conquests of the forest-steppe zones to the North of Mongolia proper), but which were hard and bloody nevertheless.
Ghengis Khan was more interested in the West (Khwarazmian Empire and the like), he sent his elite forces to the Caucasus and Volga as a reconnaissance.
That was what truly interested him.
His main imperial idea was to unite "all peoples of the felt tents" (the nomads of the Great Eurasian steppe) and conquer the lands immediately neighboring this steppe.
Even the first Grand campaign after Ghengis Khan's death (definitely in accordance with his last will) was to the West, through the Great Steppe, up to Hungary.
Contrary to popular belief, China was not priority for Ghengis Khan, though it was close at hand.
Maybe he considered it a "sitting duck" and left it to be conquered by his sons and grandsons...
Who knows...