The Viets spent 1076 preparing defenses. It is recorded that, in late spring of this year, the 10-year-old king was taken to inspect a “water barrier,” which, considering the course of later battles, was likely an underwater obstacle sealing the Bach Dang estuary. The naval war began three months before major land battles and continued for two months after withdrawal of the Song army. No details of the naval fighting survive except that there were “many sea battles,” that the Song navy never penetrated the Red River plain, and that Thang Long’s navy continued attacking long after the withdrawal of Song land forces. Viet supremacy at sea, demonstrated against Champa in 1044 and 1069, was guar- anteed by the crippling blow dealt to Song naval forces by Ly Thuong Kiet in 1075, from which Song had not taken time to recover before launching its attack.
Guo Kuei advanced his headquarters to Nanning in late autumn and within a month had advanced to the border opposite Lang Son. There he waited for over two months while one of his generals took a side trip to secure Cao Bang. Zhao Xie was critical of the Cao Bang campaign, believing that too much time was lost bringing the soldiers sent there back to join the main body of the expedition before advan- cing. He argued that these soldiers should be sent directly through the mountains toward Thang Long from Cao Bang, allowing the main force to press on, but his opinion was ignored. Furthermore, stubborn Viet resistance in Cao Bang prolonged the delay. Finally, near the end of the lunar year, the Song expedition, numbering around 50,000 troops and more than double that number of porters, crossed into Viet territory, pushed aside border guard units, marched down to the lowlands, and arrived at the Cau River where the Viets had prepared their defenses.
On the road from the border to Thang Long, the Cau River is the only natural obstacle between the mountains and the Red River. Around forty-five kilometers northeast of Thang Long, it is anchored upstream in the valleys leading to Cao Bang and downstream it joins its waters with other rivers at Pha Lai, beyond which lie soggy plains unsuitable for the movement of large land armies. It shielded the heartland of old Giao that was filled with Buddhist temples and the royal estates of the Ly in modern Bac Ninh Province. The Viets waited for the Song army on the southern bank of the river, where they had built multiple layers of bamboo fences and anchored over four hundred war boats. The Song army arrived on the northern bank of the river and there it stalled for lack of naval support.
Zhao Xie sent soldiers to cut down trees for building catapults and floating bridges. The Song prepared an attack at a place where a hill on the northern side of the river enabled soldiers to assemble out of sight of Viet observers perched in watchtowers on the southern bank. Song catapults are said to have sent stones falling like rain upon the Viet war boats to open a way for the attack. Several hundred Song soldiers crossed the river on floating bridges before the bridges were destroyed. They set many bamboo defense walls aflame but the walls were in so many layers that they could not break through. Song reinforcements arrived by raft and advanced a maximum of eight kilometers toward Thang Long, but Viet troops eventually succeeded in killing or capturing all the Song soldiers who had crossed the river. This turned out to be the furthest advance of the Song expedition. After more than a month of repulsing further Song efforts to cross the river, Ly Thuong Kiet was emboldened to essay a frontal assault to disperse the Song army. Under cover of night, he led thousands of soldiers across the river. The Song front line began to collapse until all the reserve forces, including cavalry units and Guo Kuei’s bodyguard, moved up and forced the Viets back into the river, capturing a Viet general and resulting in the drowning of two Ly princes.
With each side having demonstrated the best that it could do in battle but without dislodging the other, the stalemate began to work against the Song forces. Provi- sions were nearly depleted. Over half the army was dead from battle, heat exhaus- tion, or tropical fevers, and most survivors were weak from illness. Furthermore, the monsoon rains were about to begin. The Song generals wanted to withdraw but they dared not do so without some token of success. The Viets understood this, and a message from Ly Can Duc to the Song emperor was delivered to the Song encamp- ment apologizing for having violated Song territory in the past, promising not to do so again, requesting permission to send tribute and re-establish normal relations, and offering a proposal for marking a mutually acceptable border.
Guo Kuei and Zhao Xie decided to regard this as a formal submission and hastened to withdraw as their rearguard held off Viet attacks. Song forces endeavored to retain control of Lang Son and Cao Bang, but within a few months, attacking Viet forces regained Lang Son and began to put pressure on Cao Bang. Song officials used criminals to work the gold and silver mines of Cao Bang until late 1079, when the Ly and Song courts reached a settlement. The Song abandoned its claims in Lang Son and Cao Bang. The Ly returned a couple hundred remaining captives seized during Ly Thuong Kiet’s pre-emptive attack and resumed their pose as loyal imperial vassals. After further negotiations over districts at the upland extremity of the frontier in northeastern Cao Bang, a border agreement was finalized in 1088. The border drawn at that time, with minor changes through the centuries, was basically the same as the Sino- Vietnamese border today.
The war we have described was a war of mutual aggravation. The factors contributing to its outbreak emerged from the context of internal imperial politics. Although the Viets had to some extent drifted away from the imperial world after the fall of Tang, they were still connected to it, as Ly Thuong Kiet’s use of Wang Anshi’s reform legislation in his war propaganda revealed, not to mention the sharing of a single civilization. The Nung Tri Cao uprising, Ly border exuberance, and Wang Anshi’s activist inclinations shook Song policy from decades of passivity on its southern frontier. The question of drawing a border to separate regions that for centuries had belonged to the same imperial world was delayed for so long because of the rugged mountains and the screen of tribal vassals. The rise of Thang Long under the first three Ly kings, however, accelerated the dangers of an unmarked border and the resolution of border irritations became a priority for both sides.
The consciousness of a fixed border between northern and southern domains of the same imperial world is expressed in a poem that later historians attributed to Ly Thuong Kiet. It was supposedly sung in temples along the southern bank of the Cau River during the weeks of fighting to motivate the Viet soldiers: “The mountains and rivers of the south are the dwelling place of the southern emperor; the border has been fixed in the book of Heaven; how dare these uncouth rebels come in for plunder; you will all go out to see their defeat and ruin.”
If this story is true, it is unlikely that the poem was sung as it exists today, for it is written in Literary Chinese following Tang-style prosodic rules, which would probably not have been wholly intelligible to Viet soldiers. Furthermore, it is unlikely to have been written by Ly Thuong Kiet, who was neither a literary man nor known to have produced any other literary works. The story of singing in temples to give heart to soldiers on the eve of battle is very plausible, but whether it was exactly this poem that was sung and whether this poem was written in the eleventh century or at a later time are questions without irrefutable answers. Nevertheless, it is plausible to imagine that the border consciousness expressed in the poem arose in the context of drawing the Sino-Vietnamese border in the eleventh century. This poem also expresses what became a tenet of Vietnamese historiography in the thirteenth century when China was conquered by the Mongols and Thang Long posed as the last remaining outpost of civilization: a southern imperial tradition with its own mandate of Heaven. Whatever else it did, by emphasizing a border separating two imperial mandates, the poem simultaneously emphasized the connectedness, if not unity, of the two mandates as northern and southern parts of a whole.
When Ly Thuong Kiet returned from his foray across the Song border in 1076, his deeds were announced before the tombs of the three first Ly kings and Ly Can Duc’s reign title was changed to trumpet a “heroic, martial, glorious victory.” However, mobilizing to meet the impending Song invasion had been a sobering experience. While Ly Thuong Kiet trained soldiers and prepared defenses along the Cau River, Ly Dao Thanh selected a cadre of educated men to supervise the military camps, to stockpile supplies, and to staff a communications network. An edict was also published “to request straight talk.” This appeal for plain speak- ing, without fear of reprisal, was an unusual, if not desperate, call for any and all ideas. In late 1076, as Song forces were entering the kingdom, Le Van Thinh, the man selected the previous year to serve as tutor to the king, was made minister in charge of all military affairs. A few months later, as Song forces were withdraw- ing in early 1077, an exam was organized to select men with knowledge of “letters and laws.” Looking beyond the military emergency, Ly Dao Thanh was endeavoring to build a system of government capable of operating without the firm and constant hand of a vigorous adult king.
The monks were also active. In spring 1077, at the peak of fighting along the Cau River, an assembly was convened at Thang Long to recite the Nhan Vuong (Chinese Renwang) Sutra, “Sutra on the Benevolent Kings,” which was commonly recited to avert disasters. According to a popular explanation for the outcome of the war, the Song generals believed that their inability to obtain victory was due to the power of the Dharma Cloud Buddha, whose temple was not far from the battlefield. Song soldiers were sent to raid the temple and take away the statue of the Buddha, which was later abandoned in a forest as Song soldiers fled from a Viet ambush on their way out of the country. The Buddha was thought to be lost until a forest fire during the next dry season consumed all the trees in the region except for one spot of dense and luxuriant foliage in which the Buddha was found. This miracle was reported to the king, who returned the Buddha to its temple with great honors.
The Ly–Song War of the 1070s emerged from the specific problem of defining a border where none had existed before. There was aggression on both sides. The negotiations that followed the hostilities and that led to a resolution of outstand- ing issues are more typical of Sino-Vietnamese relations than are the few episodes of war during the past millennium.