The collapse of the Khitans brought about the fall of the Song Dynasty:
The Khitan emperor Tianzuo had been an awful monarch. He spent the whole time, literally every day out of the palace with his buddies hunting in the countryside. The administration and the army decayed with rampant corruption, and nobody took care to straight things out.
Now even if the Khitan ruling class were a bunch of corrupt slackers, the Khitan still should have had nothing to worry about. The Tanguts to their west was effectively a vassal state, they had them bought and controlled; and the Song down south were just not a threat. They hardly had a functional army. So the Khitans could have potentially kept being a failed state forever. Alas, they had trouble to their east.
Now to recap, the Khitans were a steppe people, and their language has been proven to be related to Mongol. So they were yet another tribe of Mongolian horse-archers.
The Khitans lived in the steppe just north of China proper, beyond the mountains that encircle Beijing today. East of that there’s the Manchurian plain, but that’s not steppe. That’s forest. You can’t herd livestock there. You can’t farm it either, it’s freezing cold. The Manchurian plain was the land of, well, the Manchus, but they were called Jurchen back then. The Jurchen came in many varieties, some more civilized than others. A branch of the Jurchen (called by yet another name) had established the Bohai kingdom way back, but the Liao had destroyed it back in the 920s. So all that expanse of land was now mostly Jurchen tribes, down to the border with Korea. The Khitans set up a series of fortresses to keep them quiet, and extorted regular tribute in form of fish, fur, falcons, and other local produce.
Again, the Jurchens weren’t herders. They were hunter-gatherers. Very efficient hunter-gatherers. They fished a lot. They hunted all the time. Steppe riders make strong armies because they are very good horse-archers. They ride all the time, and they hunt a lot. But Mongols hunt for sport, they eat domesticated livestock. They don’t really need to hunt. The Jurchens didn’t have a choice: hunting was how they good their food. They practiced every single day. The Jurchens were such good archers that the Khitans report being amazed at how a Jurchen man could shoot a target 200 meters away.
The Jurchens were a bunch of unruly tribes who hated each other, but in recent years the Wanyan clan had risen and subdued most of the Jurchen tribes in the area where today the city of Harbin is. Now, in normal circumstances the Khitan court shouldn’t have allowed that to happen. It’s easier to control a bunch of divided tribes than a unified ethnic group. The Wanyan clan worked case by case, sent spies and sycophants to court, raised the tribute sent to the Khitan aristocrats, and basically sold them that it was in their best interest that the Wanyan owned the whole area so they could better control the locals and produce fur and falcon more efficiently. The Khitan were very much into fur and falcon; you didn’t get far in Khitan aristocracy if you didn’t have a falcon to hunt. And the Jurchens also sent troops to help the court crack down on an internal rebellion by a Khitan rebel. And so the Khitan court stupidly look the other way while the Wanyan started what was in retrospect an obvious case of state formation of the Jurchen people.
Eventually the Jurchens founded a good excuse and rose in rebellion against the Liao. Remember the Jurchens were a hunting people. The rebelling army was at most 2500 men, while the Liao had hundreds of thousand of soldiers at their command. But premodern war isn’t a question of numbers. The Jurchens slowly grew by targeting other ethnic groups which were discontent with the Khitan. When the Khitan sent a 100,000 army to quash the rebellion, the Jurchen knew all about it. They assaulted the camp at night with 3,000 men, and killed everyone on sight, completely destroying the Khitan army.
Asymmetric motivation is a very powerful force in history. The Jurchens were few, but they were strong, and they were fighting for their people, to build a state and lord over others. They fought willingly because they had little to lose, and lots to gain. The Khitan soldiers had very little reason to risk their necks in fighting the Jurchens. Fighting China is fun, you get into China, kill a bunch of wimps, and get to grab their stuff. Silk, pots, gold, women! But fighting the fierce Jurchens? What are you gonna grab from them? Smelly leather boots? The Khitan were no match for the Jurchens, who in no time took over all the fortresses in the Manchurian plains, and were dangerously close of the Khitan capital.
News of the Khitan troubles got to China’s capital. Our artist emperor was of course ecstatic. At last! We should take advantage of that. All the sycophantic ministers proposed making an alliance with the Jurchens. Let them take all the barbarian land they wanted, in exchange of the Song taking back the northern edge of the Chinese plain and the mountain passes. The Jurchens agreed, but stipulated that the Song had to take the land they wanted by themselves. The Jurchens weren’t going to do the job for them. Thus a formal alliance was achieved.
The whole thing stunk. For better or worse, Song China and the Khitan Liao Dynasty had been in peace for 100 years. The Khitans could’ve kicked Chinese ass any time they wanted, but they respected the treaty. Now that the Khitans were in trouble, the Chinese didn’t wait a minute in betraying the treaty and stabbing them in the back. That wasn’t a very nice thing to do. It wasn’t very smart either.
Nobody told Huizong that, though, who was still having fun playing soccer and visiting hookers through his secret tunnel. In 1121 He ordered his closest eunuch, Tong Guan, who is famous as the only bearded eunuch in Chinese history, to command 150,000 troops and go straight to the southern capital of the Khitans, what is today Beijing. The Khitans in their steppe homeland were running from the Jurchens as fast as they could; surely they wouldn’t hold in the south very long either.
But the Chinese were still just no match for the Khitans. The Khitan commander in the south, Yelu Dashi, who was also perhaps the most incredible heroes in this story, held the walls, struck back at the Song forces, and destroyed the whole army. 150,000 men, gone. The whole Song army vanished in what was supposed to be a cakewalk. The eunuch commander panicked. He couldn’t just go back and say he didn’t take the land! They execute you for that stuff. So he sent an envoy to the Jurchens, saying: “Hey, we’re having some trouble here conquering the city. Why don’t you come down yourselves and take it, in exchange you can have all the booty: the gold, the women, the children, take them all. We’ll pay for all supplies you need. After you’re done you leave and we’ll take the land as agreed, right?”.
Well, why not. The Jurchens found it to be a good deal, so they came back through the mountain passes, and conquered Beijing in a week. Grabbed the gold and valuables, took the local women as concubines, took the children as slaves, sent them back to the Jin capital, close to today’s Harbin.