Cao Wei’s invasion of Shu and the Chen Cang floods are a turning point in a campaign that begins with the prospect of overwhelming force but ends with a withdrawal exposed to counterattack. Facing an army of four hundred thousand, Kong Ming predicts that prolonged rain will make the mountain approaches impassable and wear down the invaders; when the flood forces Cao Zhen’s troops to retreat, Kong Ming uses their expected caution against them, positioning Shu forces to threaten their route through the Qishan hills.

Background and Strategic Stakes

The invasion follows the earlier defeat at Jieting and the withdrawal of Kong Ming’s army. In the autumn of the next year, Cao Zhen proposes an attack directly on Hanzhong, seeking to seize the mountain pass into Shu. The plan receives imperial approval, and Cao Zhen is ordered to advance with Sima Yi and Liu Ye at the head of four hundred thousand troops. The scale of the force makes the threatened pass a strategic bottleneck: if Wei can take it, the road toward Hanzhong is open.

Kong Ming responds by summoning Zhang Ni and Wang Ping and ordering them to defend the pass with only a thousand men. Their doubts are understandable given the disparity in numbers. His decision rests not on matching the invading army in strength, but on a forecast that changes what strength can accomplish: he expects rain to make an advance through the mountains difficult and to undermine the morale of the Wei troops. While the small force holds the route, Kong Ming prepares a larger army to strike when the invaders withdraw.

The Flood at Chen Cang

Sima Yi reaches Chen Cang and finds that the city has been razed, leaving no shelter. His commanders urge him to seek refuge elsewhere, but Sima Yi recognizes the same danger Kong Ming has anticipated: the stars foretell torrential rain. He orders the army to remain among the ruins and build bivouacs there rather than move to the plains or mountains.

The storm lasts for many days. Water rises more than three feet through the encampment, and the soldiers remain cold and wet; horses die, illness spreads, and morale collapses. Once the imperial order arrives to abandon the campaign and return home, the troops welcome it, though they know that Shu may attack during the retreat. The flood thus does more than obstruct an advance. It turns the planned invasion into a difficult withdrawal, with the Wei army weakened and alert to the danger behind it.

Turning the Retreat into an Ambush

Kong Ming anticipates that Sima Yi will leave troops behind to ambush any Shu force pursuing the retreating army. Instead of chasing the Wei troops into a prepared trap, he sends his forces ahead to occupy the pass through the Qishan hills, where they can threaten the retreat itself. Wei Yan, Chen Shi, and others are assigned to guard the pass, with orders to advance cautiously and watch for ambushes.

The plan depends on discipline as much as prediction. At Qishan, Chen Shi grows impatient with Wei Yan’s cautious approach. He argues that the rain-weakened Wei soldiers will not be able to mount an effective counterattack, and breaks away with five thousand men. The force is ambushed; Chen Shi loses most of his men before retreating back into the pass with only a small, wounded remnant. His failure demonstrates the risk Kong Ming’s orders were designed to manage: an opportunity can become a trap when a commander substitutes confidence for coordination.

The Counterattack and Its Consequences

Cao Zhen’s camp has grown lax because the Shu army has not appeared. A force of five thousand is sent out to investigate, but Kong Ming’s men ambush it, killing its commander and compelling the troops to surrender. Kong Ming then uses the captured Wei armour and clothing to disguise Shu soldiers as Wei troops. Sent toward Cao Zhen’s camp, they deceive the defenders and help open the way for a mass attack that destroys the camp and scatters its forces.

Cao Zhen narrowly escapes, rescued by Sima Yi. The two commanders have already disagreed over whether Kong Ming would lay an ambush at Qishan: Sima Yi expects one, while Cao Zhen does not. After the defeat, Sima Yi proposes that they set aside their rivalry and serve Wei together. The campaign therefore produces not only a failed invasion and a Shu counterstroke, but a change in the relationship between the Wei commanders, who must cooperate after their competing judgments have been tested.

Kong Ming orders Chen Shi executed for disobedience but spares Wei Yan, whom he intends to use in the future despite not fully trusting him. The distinction reinforces the importance the episode places on obedience to a coordinated plan, while leaving Wei Yan’s position unsettled. After learning that Cao Zhen is ill, Kong Ming seeks to weaken the enemy’s morale further by sending him a letter through some of the surrendered Wei soldiers, whom he releases. The flood has ended the invasion; the subsequent deception and ambush extend its effects into the retreat, where weather, terrain, intelligence, and command discipline continue to shape the outcome.