After the Mongol Yuan and southern Song dynasties jointly destroyed the Jin Dynasty, the two sides maintained peace for a short period of time. In 1234, on the tenth day of the first lunar month of the first year of the Southern Song Dynasty, the Southern Song Dynasty and the Mongols invaded Caizhou, and the Jin Kingdom fell. According to the prior agreement, the two countries withdrew separately, and Henan became an unoccupied area. In May, the Southern Song Dynasty sent troops to retake Kaifeng and Luoyang, and according to the Yellow River as a defensive line, history called "Duanping into Luo". As a result, the Mongols counterattacked, and coupled with the lack of grain and grass, the Song army collapsed back to the south.
After this battle, the Southern Song Dynasty and Mongolia completely tore their faces, and the two countries entered a state of war. Strictly speaking, this action in the Southern Song Dynasty has the meaning of default. However, Kaifeng and Luoyang were the former capitals of the Song Dynasty, and if they could be recovered, they would not only inspire the people's hearts, but also push the defensive line to the Yellow River, greatly expanding the strategic depth. Therefore, whether in terms of symbolic significance or practical interests, this action is beyond reproach. Moreover, based on the personality of the Mongols, the invasion of the south was sooner or later, so it was possible to make strategic arrangements in advance. But the problem was that the plan of action was not well planned and was so hasty that the grain and grass were defeated and the loss of a large number of troops was also lost.
Since the war between the two countries has begun, there is nothing to say, so let's start a war when we are ready. As a powerful side, the Mongols naturally had to make a plan first and how to attack. The Mongols were quite highly skilled militarily, both strategically and tactically. After a long war with Jin, the Southern Song Dynasty set up three defensive zones from east to west, which were the Lianghuai Defense Line, the Huguang Defense Line, and the Sichuan Defense Line. After some consideration, the Mongols chose the main direction of attack in Sichuan, assisting in the direction of Huguang and Yunnan, and for this reason, they also deliberately launched a military operation to conquer Nanzhao.
Sichuan was chosen as the main direction of attack, first, because Sichuan has basically not been damaged by war since the unification of the Northern Song Dynasty, and it is an important source of wealth for the Southern Song Dynasty. It was the most economically developed area in the Southern Song Dynasty, and the annual tax on daily necessities such as tea, salt, cotton and other daily necessities accounted for two-fifths of the country, the liquor tax was 40 percent, and the military salary was one-third of the total. If Sichuan can be occupied, it will be a huge blow to the Southern Song Dynasty, almost a bone-crushing blow. In addition, during the southern Song Dynasty's long war with Jin, Lianghuai, as an important defensive line for the Gongwei Jingshi, was taken care of by the government and was built into a solid defensive line, and it was not easy to break through it.
Mungo
Choosing Sichuan as the main direction of attack is probably also the inspiration that the Mongols got from history. The strategy adopted by the Jin Dynasty to eliminate Wu was to go down the river from the upstream. Meng Yuan also wanted to occupy Sichuan, following the example of Wang Mao's building ship down Yizhou, down the river, straight to Jiankang. Bypass the two Huai defense lines and directly break through the Yangtze River defense line.
In 1234, the two countries exchanged fire, but because the main direction of the Mongol attack was in the west at that time, the war with the Southern Song Dynasty was not expanded. Subsequently, the Mongols were caught up in a struggle for the Khan's throne for more than a decade, so it was not until Möngke obtained the Khan's throne that a large-scale conquest of the Southern Song Dynasty began.
In 1258, Möngke ordered an all-out war against the Southern Song Dynasty. He personally led the main force to attack Sichuan. His brother Kublai Khan attacked Huguang, and the Mongol army in the direction of Yunnan responded. However, the Mongols were not good at attacking the city and pulling out the village, and were resisted by the Song army step by step, and the progress was slow, and finally they were blocked by the Fishing City of Hechuan in Chongqing and fell into a protracted war. By August 11, 1259, Möngke, who had been frustrated under the Diaoyu City, fell ill and died (said to have been killed by the Song army's trebuchet), and the Mongol army was forced to withdraw. On the other hand, Kublai Khan on the Huguang side crossed Xiangyang to the south, but was hindered from crossing the river under the city of Ezhou, and after learning of Möngke's illness and death, reached a peace agreement with jia Xiangdao, the commander of the Southern Song front, and returned to the north to compete for the Khan's position. The Yunnan army was also hindered by Tanzhou and returned to the north with Kublai Khan's reception. In this battle of the Song Dynasty, the Mongol side was blocked in three places, and Möngke was killed, which can be described as a complete defeat. But did the Southern Song Dynasty win? Their losses were even greater, and Sichuan, which was already in ruins, was completely scrapped after this battle, and the Southern Song Dynasty suffered heavy financial losses, making the future anti-Yuan war more difficult.
Kublai Khan
After Kublai Khan seized the Khan's throne, he listened to the advice of the Southern Song general Liu Quan and focused his attack on the middle road. Sichuan was already in ruins, and the remaining fortresses were not easy to attack, so they simply abandoned them, just like the two Huai defense lines of the Southern Song Dynasty, leaving a small number of troops to monitor. In 1268, the Yuan army attacked Xiangyang, and after six years of siege, Xiangyang finally fell in 1274, and the Huguang defense line of the Southern Song Dynasty was torn open. Subsequently, Kublai Khan ordered the Yuan army to launch a general attack on the Song, the reinforcements entered the Yangtze River, down the river, and continuously broke the Song army, in 1276, Lin'an fell, the general trend of Yuan unification was determined, and all that was left was to deal with the remnants of the Southern Song Dynasty everywhere. After decades of continuous war, the Mongol Yuan finally destroyed the Southern Song Dynasty, but compared with the conquest of other regions, the attack of the Southern Song Dynasty can be said to be the most difficult conquest of the Mongols.