At that time, Đại Việt was a formidable country, despite being much smaller than the Yuan empire. Instead of attacking Đại Việt directly, the Yuan decided to target its smaller neighbor, Champa, first. Champa was a collection of Cham states along the coast of present-day central and southern Vietnam before its eventual annexation by Vietnam. Its strategic location made it an attractive target; if the Mongols could conquer Champa, they would gain a key advantage in their campaigns against Đại Việt, Chenla (the Khmer Empire), and other southern regions.
After defeating the Song Dynasty, the Mongols sent emissaries to Champa, demanding that King Indravarman V submit and come to the Yuan court. When the Champa king ignored these demands, the Yuan prepared to invade. They also demanded that Đại Việt allow their army to pass through its territory, provide supplies, and assist in the conquest of Champa. However, Đại Việt found ways to refuse. Concerning weapons and troops, the rulers stated that their weapons were worn out from disuse, and their soldiers had returned to their homes. Regarding food supplies, they claimed natural disasters had left them with insufficient provisions for themselves. To avoid further pressure, Đại Việt promised to send food to Ung Châu, an unstable region with many uprisings against the Yuan. Knowing this was a delay tactic, the Yuan general Ariq Qaya was angered but found no way to retaliate.
In reality, Đại Việt’s economy had recovered quickly from its earlier war with the Mongols, and the kingdom was flourishing. Instead of aiding the Yuan, Đại Việt sent 20,000 soldiers and 500 warships to support Champa. The rulers of Champa had also anticipated the Yuan’s ambitions and prepared for war. In 1282, when the Yuan court sent envoys to other southern countries demanding submission, Champa captured all the envoys passing through its territory, knowing that Yuan control would be disastrous during the coming conflict.
Later that year, a massive Yuan force arrived from Guangzhou. Despite repeated demands for submission, Champa resisted and even sent a provocative letter to the Yuan general Sogetu, leading to the start of the invasion. However, before the first battle, stormy seas damaged much of the Mongol fleet, though few casualties were reported. The Yuan forces attacked at night, and after half a day’s battle, Champa’s Prince Harijit ordered a retreat, joining forces with the king in Vijaya, the Champa capital.
Sogetu then marched toward Vijaya, where Champa’s leaders used clever diplomacy to buy time for their forces. They pretended to negotiate submission, sending envoys with tribute and delaying tactics. The Yuan forces entered an empty Vijaya, falling into a trap set by Champa. Envoys repeatedly assured Sogetu that the king and prince would surrender soon, while Champa used the time to fortify their defenses and prepare a counterattack.
In 1283, the Champa ambassador, the king’s uncle, continued this ruse, claiming that the king was too ill to appear in person but promising his eldest son would arrive soon. Sogetu, unsure of what to believe, accepted tribute and delayed further action. A Chinese informant, however, revealed that Champa was secretly preparing for war, and while Sogetu was skeptical at first, the ambassador managed to maintain the Yuan general’s trust. Meanwhile, Champa organized guerrilla warfare, ambushing Yuan troops and disrupting their supply lines.
Sogetu soon realized that Champa’s forces had not disbanded and were still a threat. Despite their initial losses, Champa’s leadership turned to guerrilla tactics, focusing on small-scale ambushes targeting Yuan soldiers stealing food from the countryside. These tactics were effective, forcing Sogetu to adopt a defensive strategy. Champa’s defenses, fortified fortresses, and strategic traps wore down the Yuan forces.
By March 1284, Ataqai led a Yuan reinforcements fleet of 30,000 soldiers and several hundred ships to aid Sogetu. However, they arrived in Champa only to find Sogetu had relocated. A devastating storm wrecked most of their fleet, forcing the survivors to retreat, though many turned to banditry instead of returning home.
Sogetu, isolated and struggling for supplies, could only defend his position and rely on agriculture for food. In the meantime, Champa’s leadership avoided direct confrontation, knowing they could not defeat the Yuan army in open battle. The Yuan troops, weakened and awaiting reinforcements, were demoralized. Sogetu sent a plea to the Yuan court, suggesting that attacking Đại Việt would be a better strategy, as it would open the way to conquer Champa and other southern kingdoms.
Kublai Khan, who had long sought control of Đại Việt, initially thought that conquering Champa first would allow for a multi-pronged attack on Đại Việt. However, given the challenges of attacking Champa—severe weather, supply issues, and strong resistance—he reconsidered his strategy.
Yuan War and Đại Việt’s Resistance
After learning of Đại Việt’s involvement in the Champa-Yuan war, the Yuan sent a letter of reproach. Đại Việt denied direct involvement, responding vaguely: “Champa is our subordinate. We sympathized with them, but cannot claim it was destiny. Even a child knows what happens when the enlightened collude with the unenlightened.”
In early 1285, the Yuan-Dai Việt war erupted. Kublai Khan mobilized a massive force of 500,000 soldiers, including elite Mongol cavalry and various ethnic groups from Yunnan and beyond, all capable of fighting in diverse environments. These troops, although well-prepared with food and medicine, lacked uniform experience across the ranks.
The Yuan had high ambitions, with some of their best generals leading the charge. Prince Toghan served as Commander-in-Chief, supported by renowned commanders like Ariq Qaya (third-ranking general in the empire), Li Heng, and Omar. Their plan was to invade Đại Việt from both the north (via China) and the south (with Sogetu leading forces from Champa), aiming for a swift conquest of Đại Việt and a subsequent expansion across Southeast Asia.
However, this strategy failed to materialize. Đại Việt employed effective guerrilla tactics, ambushing Yuan forces with smaller, more mobile units, preventing the Yuan from achieving their rapid victory.
See also: Tran dynasty: the dynasty that defeated the Mongols 3 times
