The northern wall of Da Xia was the first human-made obstacle the Mongols would have to overcome before finishing their crossing of the Gobi Desert. The invading army would have been spotted by forward watchtowers; sentries would light beacons to alert roaming pastoralists, neighbouring signal towers and military commanders. Messengers were then dispatched from the garrison fortresses to alert neighbouring watches and high-level administrative centres – presumably triggering a chain of alerts which would end up in the Tangut capital at Zhongxing, or Yinchuan, in modern day Ningxia Autonomous Region.
The Tangut Law Code indicates that there were severe penalties for unapproved troop relocation around the border forts, even in the face of an enemy; and perhaps this is evidence that the wall was not primarily a defensive structure. The Law Code characterises frontier forts as nodes in an imperial network that regulated trade, collected customs duties and monitored population movements. It was staffed by men, women, auxiliary soldiers and labourers and was supported by the pastoralist families who moved through the region. The garrisons were there to handle small groups moving through the border, not to engage with large invasions – they were legally required to leave that response to the interior. The Mongols then were able to cross the frontier with ease and continue on their way.
Moving across Inner Mongolia, the Mongol army once more traversed the Langshan going through the mountain pass near Wulahai. This time, however, Da Xia was prepared for them. An Quan had dispatched the Crown Prince Chengzhen and general Gao Yi with a force of 50000 men to stop the Mongol invasion reaching the town. It failed. The XiXia Shushi states that the Tangut army suffered a major defeat, General Yi was captured and then executed for refusing to bow to Chinggis Khan.
The Mongols then invested Wulahai, which surrendered in April or May. The XiXia Shushi states that the commander of the town was advised to surrender, which he appears to have done, but when the Mongols entered the city street fighting broke out, led by the Grand Tutor of Da Xia, a man named Xibi Shi. The resistance was put down and Xibi Shi was captured.
The next stage of the attack focused on the mountains surrounding Zhongxing. The XiXia Shushi states that the Mongols besieged the Ke Yi Gate, which is described as being the outer defence of Zhongxing. It was at a place where two mountains stood opposite each other with only a narrow path in between and the terrain was so precipitous that it could not be climbed.
During normal times 7000 men would be stationed there; and despite the Mongols having been in Da Xia for a couple of months by this point, the Tangut communication network had clearly been unaffected by their arrival because An Quan was able to reinforce Ke Yi with a further 5000 men before the Mongols could reach it.
When the Mongols arrived at the gate in May or June they were immediately attacked by the Tangut army led by the general Wei Ming Linggong. They charged down the hillside towards the arriving Mongols and inflicted a small defeat on them. The Tangut army then retreated behind the gate and both sides settled in for the siege.
The siege continued until August when reinforcements arrived from Mongolia. At this point Chinggis Khan decided to try and lure the Tangut army out using the feigned retreat. The Mongol camp packed up, and a band of light cavalry was left to act as a rearguard. The rest of the army was placed in ambush.
Wei Ming Linggong took the bait and lead the garrison out to attack the retreating Mongols. He was captured and the Ke Yi Gate was breached. The Mongol army moved through the pass and put Zhongxing under siege. The defence of the capital was led by An Quan.
In September the Mongols constructed dykes to redirect the Huang He into the city, but despite the increased hardship, An Quan was able to hold out until October before seeking external aid. He appealed to the Jin for help, but his request remained unanswered and the siege continued.
By December or January the conditions within Zhongxing had massively deteriorated and the city walls were in serious danger of collapsing. Relief came when one of the dykes burst spilling water across the floodplain forcing the Mongols to lift the siege and withdraw to higher ground.
Chinggis Khan sent the imprisoned Grand Tutor, Xibi Shi, to negotiate the surrender of the city. To retain any leverage in negotiations, An Quan refused to allow Xibi Shi entry into Zhongxing where he would have discovered the desperate condition of its inhabitants. Instead, he talked to the Mongol embassy from the walls and a peace treaty was agreed where the Tangut would provide the Mongols with camels, hunting birds and a princess. There was also a promise of military aid if the Mongols requested it, but it came with caveats.
The Mongols waited around until the camels had been collected and then took all of their loot back north, leaving a token force at Zhongxing to ensure the good behaviour of the Tangut. In April 1210 the general Wei Ming Linggong, a prisoner since the fall of Ke Yi Gate the previous August, was released from captivity in Mongolia.