Cao Zhen attacks Shu, meets heavy rain, withdraws army

Cao Zhen’s invasion of Shu ends in retreat when torrential rain overwhelms the Wei army at Chencang. The campaign turns on a contest between military strength and the conditions that strength cannot command: Kong Ming reads the coming weather and prepares to exploit the withdrawal, while Sima Yi recognizes the forecast yet finds his army’s position untenable after a month of flooding.

Background and Objectives

In the autumn following Kong Ming’s withdrawal to Chengdu to recover from illness, Wei seeks to reverse the balance and invade Shu. Cao Zhen, restored to health and eager for revenge, proposes a direct advance against Hanzhong. The court approves the plan, and Cao Zhen is ordered to join Sima Yi and Liu Ye with four hundred thousand troops. Their objective is to seize the mountain pass into Shu and attack Hanzhong, a large-scale offensive aimed at opening a route into the rival state.

The expedition’s size makes the campaign appear to favor Wei, but the route and its exposure to weather are central to what follows. The army must operate in mountainous terrain, and the rain that Kong Ming expects will make an advance up the mountains impracticable. The invasion’s strength is therefore vulnerable to the conditions of its approach: the terrain that offers access to Shu also magnifies the cost of prolonged bad weather.

Kong Ming’s Preparation

In Chengdu, Kong Ming learns of Wei’s movements and summons Zhang Ni and Wang Ping. He orders them to defend the pass with only a thousand men. They doubt that so small a force can stop an army of four hundred thousand, but Kong Ming bases the assignment on his observation of the night sky. He predicts that a massive rainstorm will last many days, weaken the invading troops, and make an attack through the rain-soaked mountains impossible.

His plan is not simply to hold the pass. Kong Ming tells the commanders that he will prepare Shu’s main force for the Wei army’s eventual retreat, when the withdrawing troops can be attacked. This distinction makes the weather forecast operational: the thousand defenders are not expected to defeat the whole invading army in open battle, but to deny it an effective advance while the larger Shu force prepares to act against its withdrawal.

The Storm at Chencang

Sima Yi reaches Chencang and finds that the city has been razed, leaving no shelter. He, too, has observed that the stars foretell torrential rain. Despite his commanders’ urging, he orders the army to remain and build bivouacs in the ruins rather than seek shelter on the plains or in the mountains. His decision responds to the same warning Kong Ming has used, but it does not remove the army from the storm’s effects.

The rain lasts for a month. Floodwater rises to more than three feet, washing away the bivouacs and leaving the troops cold, ill, and persistently wet. Horses die, and morale collapses. When an imperial command orders the expedition to abandon its attempt and return home, the soldiers welcome the decision, even as they recognize that the retreat exposes them to a possible Shu counterattack.

Retreat and Strategic Significance

The Wei army turns back slowly, with its troops continually afraid that Kong Ming’s forces will attack. The account makes the withdrawal itself the next strategic problem: Wei has been compelled to abandon its objective, but the route home may still expose it to the opponent’s prepared response. The episode ends on that unresolved expectation, asking what Kong Ming intends to do next rather than yet narrating the outcome of his plan.

The contrast between Kong Ming and Sima Yi is not that one foresees the weather and the other does not. Both recognize the forecast; the difference lies in how the coming storm fits into their plans. Kong Ming links the rain to the defense of the pass and to a later opportunity against a retreating army. Sima Yi’s decision to remain at Chencang cannot protect his troops from a month of flooding, and the resulting order to withdraw gives Kong Ming the situation he anticipated. The storm thus functions as a strategic force in its own right, changing the campaign from an invasion of Shu into a hazardous retreat without a decisive pitched battle described in this episode.