Strategic Background
As a sub-event of the Six Sorties from Mount Qi, the Shangfang Valley scheme was born of a deadlock: Zhuge Liang had pushed deep south of the Wei River, only to face Sima Yi's turtle-shell tactic of "holding fast and refusing to give battle." The Wei army was waiting out the standoff for circumstances to change, betting that the far-campaigning Shu army's grain supply would falter and that its weary troops would withdraw of their own accord. For Zhuge Liang, the only way to break the deadlock was to fashion a bait Sima Yi could not refuse—since the opponent would not answer a challenge, he had to be made to attack of his own will. The whole Fire Attack at Shangfang Valley was, in essence, an "anti-holding-fast" design aimed at the enemy commander's character and judgment.
Dispositions of the Two Sides
The Shu army's dispositions formed a multilayered killing net. Ma Dai was put in charge of the engineering and the hidden fires: building wooden palisades, digging deep trenches, scattering dry firewood and landmines throughout the valley, setting up dummy shelters on the hills, and using a seven-star banner and bright lanterns as signals [c:16071]. Wei Yan served as the bait, leading five hundred troops in a feigned defeat to draw Sima Yi toward the seven-star banner and the lanterns [c:16071]. Gao Xiang, meanwhile, repeatedly made a show of ferrying grain with the Wooden Ox and Flowing Horse, creating the illusion that "the Shu army's provisions were stockpiled at Shangfang Valley" [c:16072]. On his side Sima Yi was led to the conclusion that "Kongming is not at Mount Qi, and the grain is at Shangfang Valley," and settled on the calculation of "feigning an attack on the Mount Qi base while personally taking Shangfang Valley to burn the grain" [c:16072]—he believed he was turning the scheme against its author, when in truth he was walking step by step into the ambush.
Key Decision Point
The deciding move lay in Sima Yi's single thought of "whether to enter the valley mouth in person." He first sent men in to scout, who reported that there was no ambush and that the hills held only thatched huts, so he judged this to be a grain depot and confidently drove his men and horses in a great mass into the valley [c:16073]. The fatal flaw in this judgment was that he equated "seeing no ambush" with "there being no ambush," overlooking that the thatched huts and dry firewood were themselves the weapon. When the torches dropped all at once and the fire-arrows and landmines went off together, the Wei army had no way to flee, and Sima Yi at one point embraced his two sons and wept aloud, believing all three of them would die there [c:16073]. From the standpoint of planning, Zhuge Liang's killing trap was nearly perfect—the variable of failure lay not in human design but in the timing of Heaven.
Outcome and Resonance
A sudden rainstorm doused the fire that filled the valley; the landmines did not detonate, the incendiaries proved useless, and Sima Yi seized the chance to fight his way out and flee back north of the Wei River, burning the pontoon bridge behind him to hold the northern bank [c:16073]. Watching from the hills as the fire rose and the rain came, Zhuge Liang sighed deeply, "Man proposes, but Heaven disposes; it cannot be forced" [c:16074]. This engagement thus became one of the most fate-laden passages in the whole book: it pushed Zhuge Liang's ingenuity to its limit while using a single rainfall to proclaim the tragic keynote that Shu Han's Northern Expeditions were "no fault of the campaign, but truly a matter of Heaven not granting him years," setting the emotional stage for the falling star at the Wuzhang Plains that soon followed.
Counterfactual Speculation
It must first be noted that this is a fictional episode invented by the Romance; there was no historical burning of Shangfang Valley. Even within the novel's own logic: had there been no rainstorm, Sima Yi and his sons would have been incinerated, Cao Wei would have lost its entire western-front command, and the course of the Northern Expeditions—and even the later succession of the Sima clan to the Wei throne—would have been utterly rewritten. But the novel deliberately arranges the rain to save the day precisely to express a historical fate beyond human power—Zhuge Liang reckoned out Sima Yi's character, judgment, and greed for merit to the last, yet could not reckon with Heaven. This "failure on the verge of success" is so moving precisely because it compresses the tension between "doing all that man can" and "submitting to the will of Heaven" into the smoke and fire of a single valley.