歴史を面白く学ぶコテンラジオ （COTEN RADIO） · History
【67-5】防ぎようなくない？城壁も補給路もぶち抜くモンゴル長期包囲戦【COTEN RADIO モンゴル帝国襲来編5】
Kublai Khan's Mongol forces sealed off the twin Song fortresses of Xiangyang and Fancheng with dozens of kilometers of siege walls, then shattered their defenses with the Muslim-engineered counterweight trebuchet known as the Huihui Pao (回回砲). This episode traces the fall of Southern Song, the founding of the Yuan dynasty, the construction of a 2,500-kilometer grand canal, and the Kamakura shogunate's decision to ignore three successive Mongol envoys as Japan stumbled toward the coming invasion.
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What this episode answers
Why did the Mongol army choose to encircle the cities with dozens of kilometers of siege walls rather than assault them directly?
The Mongols encircled Xiangyang and Fancheng with dozens of kilometers of siege walls and moats to sever all supply routes, rather than assaulting directly, using labor and engineers conscripted from conquered territories like Goryeo to build this unprecedented attacker's wall.
What was the Huihui Pao, and how was it able to breach the formidable walls of the Southern Song fortresses?
The Huihui Pao was a counterweight trebuchet whose technology the Mongols acquired from conquering the Abbasid Caliphate; using centrifugal force like a giant fishing rod, it hurled roughly 90-kilogram stones that could blow a two-meter hole in a wall with a direct hit, demolishing Xiangyang's defenses after four months of bombardment.
Why did Kublai rename his state 'Yuan' and undertake the construction of a new capital and a grand canal?
Kublai renamed his state 'Yuan' and built the new capital Dadu (on the site of present-day Beijing) along with a grand canal in order to link Dadu with the material wealth of the Jiangnan region, since the 2,500-km canal network let 3 million koku of rice flow into Dadu annually, providing the logistical backbone for Beijing to function as the capital.
What did the Goryeo Sambyeolcho write to Japan, and how did Japan respond?
The Sambyeolcho wrote to Japan warning that the Mongols would certainly attack and asking Japan to send rice and military reinforcements so they could hold the line; Japan responded with bewilderment, since Goryeo had just been arriving as a Mongol envoy, and sent no reply.
What domestic circumstances lay behind the Kamakura shogunate's refusal to send any reply to the Mongol envoys?
The shogunate refused to reply partly because war was seen as coming regardless, making a reply pointless, and domestically it was consumed by disorder from armed 'akutō' forcibly seizing vassal lands and occupying ports, with governance over Kyushu vassals especially weak and unreliable compared to the Kanto heartland.
Turning the Tables — the Attacker Builds the Wall: the Siege of Xiangyang and Fancheng
Rather than assault the two great Southern Song cities of Xiangyang and Fancheng head-on, Kublai Khan's Mongol army chose a strategy of total encirclement. Using the hills to the south as an anchor, they constructed dozens of kilometers of siege walls and moats that completely severed all supply routes into both cities. Walls had always been the defender's weapon, yet here the Mongols threw up an outer ring of earthworks on an unprecedented scale as the attacking force. This feat of military engineering was made possible by the skilled laborers and engineers conscripted from conquered territories such as Goryeo.
Southern Song threw its full national strength into a relief army of 100,000 troops, but Mongol intelligence detected the movement in time, and an equivalent force was dispatched to intercept it. The relief army was defeated and its commander captured. Even so, the two cities held out for another two years.
The Huihui Pao — Punching a Two-Meter Hole Through the Walls
The stalemate was finally broken by the Huihui Pao (回回砲), a counterweight trebuchet whose technology the Mongols had acquired during their conquest of the Abbasid Caliphate. The weapon worked like an enormous fishing rod, using centrifugal force to hurl stones of roughly 90 kilograms over great distances; a direct hit could blow a two-meter hole in a city wall. The Mongols first stormed and took Fancheng, then moved the Huihui Pao inside and pounded Xiangyang for four months. Defensive structures were demolished one after another, terror spread among the civilian population, and Xiangyang fell. From that point on, Southern Song was incapable of mounting any meaningful counteroffensive.
The Founding of Yuan, Dadu, and the Grand Canal
In the course of conquering Southern Song, Kublai formally renamed his state 'Yuan' (元) and built a new imperial capital called Dadu (大都) — on the site of present-day Beijing. To link Dadu with the material wealth of the Jiangnan region, the Yuan dynasty excavated 1,700 kilometers of new canal. Combined with existing waterway sections, the total length reached 2,500 kilometers — equivalent to the distance from Hokkaido to Kagoshima — and the entire network was completed within 30 years. Through this grand canal, 3 million koku of rice flowed into Dadu annually, providing the logistical backbone that allowed Beijing to function as China's capital.
The Goryeo Sambyeolcho Rebellion and the Appeal to Japan
In Goryeo, an elite special force of the military government known as the Sambyeolcho (三別抄) rose in rebellion against submission to Mongol rule. They sent a letter to Japan with an urgent message: the Mongols would certainly attack Japan, and if Japan would send rice and military reinforcements, the Sambyeolcho would hold the line. Japan's reaction was bewilderment — Goryeo had been arriving as a Mongol envoy just moments before, so why was it now fighting the Mongols? Japan sent no reply. The Sambyeolcho rebellion was suppressed by a combined Mongol–Goryeo force, and Japan's puzzled non-response changed nothing.
The Shogunate Ignores Three Mongol Envoys — and the Disorder at Home
Throughout this period the Mongols dispatched envoys to Japan repeatedly. The third envoy, Zhao Liangbi (趙良弼), arrived at Hakata with a retinue of 100 men and attempted to deliver the state letter directly. The imperial court in Kyoto had begun deliberating over a written reply when the Kamakura shogunate intervened: since war was coming regardless, a reply was pointless. Zhao Liangbi left without ever meeting the emperor or the shogun, and Japan had now ignored the Mongols three times.
Meanwhile, domestic conditions were deteriorating. Armed strongmen known as 'akutō' (悪党) were seizing lands from the shogunate's retainer vassals by force — occupying ports and simply declaring them their own — and even the retainers and power-holders were intertwined with these outlaws. Governance over the Kyushu vassals was especially fragile compared with the Kanto heartland; many lords were based far away and could not be reliably reached by orders from above. The shogunate faced the Mongol threat while simultaneously scrambling to put its own house in order — a house that was far from ready.
Takeaways
- The Mongol army won the prolonged siege of Xiangyang and Fancheng through a combined strategy of civil engineering and weaponry — encircling both cities with dozens of kilometers of siege walls and then breaching their defenses with the Islamic-derived Huihui Pao trebuchet.
- After founding the Yuan dynasty, Kublai completed a 2,500-kilometer grand canal in 30 years that shipped 3 million koku of rice annually to Dadu, creating the logistical infrastructure that made Beijing viable as China's capital.
- Japan ignored Mongol envoys three times, and was in no position to respond effectively: the akutō lawlessness and the weak governance of the Kyushu vassals left the country dangerously unprepared for the invasion to come.
This article is edited from a machine transcript of the first 30 minutes of the episode. It is not an official summary — please listen to the episode itself.