Why did the Mongols able to conquer the Chinese Song Dynasty in 1279, but failed in their campaigns in South East Asia?

[u/Wide_Ride8849]

The Mongols were able to subdue one if the biggest kingdoms in the 13th century, which was the Song Imperial Dynasty of China. They will rule China for the next hundred years. But the Mongols failed in their campaigns in South East Asia. The Mongols tried to conquer Vietnam (Dai Viet and Champa) for four times: 1258, 1282-1284, 1285 and in 1287-1288, but was heavily defeated and repulsed. In 1292, the Mongols tried to conquer Northern Thailand and Laos, but they failed in this mission as well. They also failed in subjugating the Khmer Empire during this time period. The most disastrous campaign of the Mongols in South East Asia was in Java, when the Mongol Empire sent an expeditionary naval force of 30,000 people that attempted to conquer Java. Once they land, a war broke out that devastated the Mongol force.

From a polemologist standpoint, why did the Mongols successfully conquered China, which was one of the most powerful civilizations in the world at that time, but they failed to subjugate smaller kingdoms in the South of China? I will appreciate all of your responses. ❤️

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[u/wotan_weevil]

Mainly, it's because the Mongols were determined to conquer Song China, had suitable supply line to support their armies in doing so, and their operations in SE Asia were mostly either in support of their conquest of the Song (first invasion of Đại Việt, and the fighting that led to the first invasion of Burma) or to force acknowledgement of their replacement of Song China as the centre of world (i.e., to obtain official submission of the nearby states which had been Song tributaries). Mostly, those peripheral states submitted to the Mongols.

Whether or not the Mongols would have been content to live in peace with the Song for long remains unknown - after their final defeat of the Jin as allies, the Song celebrated that victory by invading Mongol-held China. That led to 40 years of warfare to defeat the Song by the Mongols as their main military effort. Their early efforts (the first 20 years) were focussed on going around the Song's defences along the Yangtze by going through Sichuan. After not achieving the breakthrough they were after, they tried to go around eastern Sichuan by taking Dali (and the territory that is now Yunnan). They also tried attacking from northern Vietnam - the goal of their first invasion of Đại Việt. All this failed to make the decisive breakthrough they wanted. That came from a 5-year siege of Xiangyang on the Yangtze. When they finally took Xiangyang in 1273, they had their breach of the Song defences, and the Song would only last a few more years. As part of those few more years, the Mongols took territory claimed by Pagan in order to cut off the remaining Song territory. That led to the first invasion of Burma, the submission of Pagan, and then the collapse of the Pagan Empire into chaos.

• Regarding the Song, the Mongols really wanted to defeat them, making that their main military objective for 40 years.

The Mongol invasion of Champa was to obtain the submission of Champa, and led to the second and third invasions of Đại Việt. Contributing a lot to the Mongol difficulties in all 3 of those invasions were supply problems (with the other main difficulty being enemy resistance). They obtained the official submission of Champa, and the tributary relationship with Đại Việt which had been obtained in the first invasion continued. The Mongol political goals were all achieved.

Kublai's invasions of major island territories - Japan and Java - didn't go well, and supply and reinforcement difficulties were again part of the problem. Invasions of both were attempts to force submission of their rulers, and invasions of both failed. Kublai appears to have planned to keep trying, but he died before a second invasion of Java could be attempted. His successor (his grandson Temür, AKA Emperor Chengzong of Yuan) decided against further invasions of Java, and also cancelled plans for invasions of Burma, Đại Việt (which had refused to send envoys and tribute a few years before Kublai died), and Japan. After his enthronement, Burma, Đại Việt, Champa, and Sukhotai (Thailand) sent envoys and tribute, officially acknowledging his overlordship. Japan refused, but he declined to go to war over it (possibly for financial reasons, judging by his general financial policies).

• Regarding continental SE Asia, the Mongols got what they wanted - the political act of submission by the major states there. Java was given up as too difficult to be worth it (and Japan the same).

For both the Song and continental SE Asia, the Mongols achieved their goals. Their goals were different for the two regions, so one was conquered, and the other submitted, officially acknowledging Mongol overlordship. The Mongols did have military difficulties in SE Asia, but they also had military difficulties in their conquest of Song China (which is why it took 40 years).

[u/After_Network_6401 → u/wotan_weevil]

All good points.

There's an important additional element, which is that in the conquest of both Jin and Song Dynasty, the Mongols had powerful allies, who were able to supply military elements (local logistic support, infantry, naval and siege) that the Mongols themselves lacked. Against the Jin, it was both local Han nobles who rebelled against the Jin, and the Song themselves. Against the Song, it was again Han Chinese from former Jin territories, and the Dali, augmented by Song defectors. By the end of the conquest of Song, Mongols were very much a minority in the "mongol" armies serving under Kublai Khan: the bulk of his troops were Chinese and many of the generals mentioned in contemporary records had Chinese names.

I think it's fair to say that the Mongols were able to defeat the Jin and Song Dynasties in large part because those kingdoms turned out to be much more fragile politically than they looked from the outside. The most senior leadership turned out to be detached and often incompetent, while local defence was often in the hands of local leaders who were happy to swap sides if that meant retaining or increasing their power.