Empty Fort Strategy

The Empty Fort Strategy is the 32nd of the Chinese Thirty-Six Stratagems. The strategy involves using reverse psychology (and luck) to deceive the enemy into thinking that an empty location is full of traps and ambushes, and therefore induce the enemy to retreat. Some examples are listed in the following sections.

Cao Cao
According to Cao Cao's biography in the Sanguozhi, in 195, the Empty Fort Strategy was used by Cao Cao against Lü Bu's forces in one incident at Chengshi (乘氏; southwest of present-day Juye County, Shandong). In the summer of that year, Lü Bu went to Dongmin (東緡; northeast of present-day Jinxiang County, Shandong) and gathered about 10,000 troops to attack Cao Cao. At that time, Cao Cao had very few soldiers with him, so he set up an ambush and defeated Lü Bu.

The Wei Shu (魏書) gave a more detailed account of the ambush. Cao Cao had sent his troops out to collect grain so he had less than 1,000 men with him in his base, which was not well defended. When Lü Bu showed up, Cao Cao sent all his available soldiers to defend the base and even ordered women to stand guard on the walls. To the west of Cao Cao's base was a dyke, and to its south was a deep forest. Lü Bu suspected that there was an ambush, so he told his men, "Cao Cao is very cunning. We must not fall into his ambush." He then led his troops to 10 li south of Cao Cao's base and set up his camp there. The following day, Lü Bu came to attack Cao Cao, and by then, Cao Cao had really set up an ambush near the dyke. Lü Bu's forces fell into the ambush and were defeated.

The "ambush" mentioned in the Sanguozhi refers to the ambush that Lü Bu's forces fell into on the second day, as described in the Wei Shu. The incident is also mentioned in the Zizhi Tongjian, written by Sima Guang in the Song Dynasty. However, the Zizhi Tongjian account, which combined the Sanguozhi and Wei Shu accounts, did not mention the events on the first day (about Cao Cao sending all his available soldiers to defend the base and ordering women to stand guard on the walls).

Debate
Yi Zhongtian, a professor from Xiamen University, commented on this incident in his book Pin San Guo (品三国) in response to criticism from Fudan University history professor Zhou Zhenhe (周振鹤) and a web user called "Hongchayangweili" (红茶杨威利), after he said in a lecture on the television programme Lecture Room that "Cao Cao's rights to the invention of the Empty Fort Strategy had been stolen from him" (referring to this incident in 195). Zhou claimed that the Empty Fort Strategy had never been used in history, so there were no "rights" to its invention; the web user argued that the incident in 195 does not count as a use of the Empty Fort Strategy.

Yi defended his claim, saying that the incident in 195 is valid, because of the circumstances under which it was used, which were very similar to the incidents involving Zhao Yun and Wen Ping (see the sections below). Cao Cao was trying to confuse Lü Bu by making use of the geographical features (the "deep forest") and by ordering women to stand guard on the walls, so as to make Lü Bu suspect that he had set up an ambush in the "deep forest" and lure Lü Bu to attack his "weakly defended" base (by deploying women as soldiers to show how "desperate" he was to set up a defence). The ploy worked because it made Lü Bu hesitate when he wanted to attack. Cao Cao had bought sufficient time to set up a real ambush, and he defeated Lü Bu when the latter came to attack again on the following day.

Zhao Yun
The Yun Biezhuan (雲別傳; Unofficial Biography of (Zhao) Yun) mentioned an incident about Zhao Yun (a general under the warlord Liu Bei) using the Empty Fort Strategy during the Battle of Han River in 219, fought between Liu Bei and his rival Cao Cao as part of the Hanzhong Campaign.

This incident took place after Cao Cao's general Xiahou Yuan was defeated and killed in action at the earlier Battle of Mount Dingjun. Cao Cao's forces were transporting food supplies to Beishan (北山) when Liu Bei's general Huang Zhong heard about it and led a group of soldiers (including some of Zhao Yun's men) to seize the supplies. Huang Zhong did not return after a long time so Zhao Yun led tens of horsemen in search of Huang. Zhao Yun encountered Cao Cao's forces and engaged them in battle but was outnumbered and was forced to retreat back to his camp, with Cao Cao's men in pursuit. Zhang Yi was in Zhao Yun's camp and he wanted to close the gates to prevent the enemy from entering. However, Zhao Yun gave orders for the gates to be opened, all flags and banners to be hidden, and the war drums silenced. Cao Cao's forces thought that there was an ambush inside Zhao Yun's camp so they withdrew. Just then, Zhao Yun launched a counterattack and his men beat the war drums loudly and fired arrows at the enemy. Cao Cao's soldiers were shocked and thrown into disarray. Some of them trampled on each other while fleeing in panic, and many of them fell into the Han River and drowned. When Liu Bei came to inspect Zhao Yun's camp later, he praised Zhao and threw a banquet to celebrate Zhao's victory.

Wen Ping
The Weilue mentioned an incident about the Empty Fort Strategy being used by a general Wen Ping around the time of the Battle of Jiangling in 223, fought between the forces of the states of Cao Wei and Eastern Wu in the Three Kingdoms period.

The Wu emperor Sun Quan led thousands of troops to attack a fortress defended by the Wei general Wen Ping. At that time, there were heavy rains and much of the fortifications were damaged. The civilians in the fortress had retreated to the fields so they could not help the defenders repair the fortifications in time. When Wen Ping heard that Sun Quan had arrived, he was unsure of what to do, but eventually he thought of using deception. He ordered everyone in the fortress to stay under cover while he hid behind the walls, creating an illusion of an empty fortress. As Wen Ping expected, Sun Quan became suspicious and he said to his subordinates, "The northerners regard this man (Wen Ping) as a loyal subject, which is why they entrusted him with defending this commandery. Now, as I approach, he does not make any move. It must be either that he has something up his sleeve or that his reinforcements have arrived." Sun Quan then withdrew his forces.

The historian Pei Songzhi commented that the Weilue account did not match the original account in Wen Ping's biography in the Sanguozhi, written by Chen Shou. Chen Shou wrote: "Sun Quan led 50,000 troops to besiege Wen Ping at Shiyang (石陽). The situation was very critical but Wen Ping put up a firm defence. Sun Quan withdrew his forces after more than 20 days, and Wen Ping led his men to attack them as they were retreating and defeated them."

Zhuge Liang
One of the best known examples of the use of the Empty Fort Strategy is a fictional episode in the historical novel Romance of the Three Kingdoms by Luo Guanzhong, which romanticises the history of the late Han Dynasty and the Three Kingdoms period. This event took place during the first of a series of campaigns (known as Zhuge Liang's Northern Expeditions) led by Shu Han's chancellor Zhuge Liang to attack Shu's rival state Cao Wei.

In the first Northern Expedition, Zhuge Liang's efforts to conquer Chang'an, a strategic city in Wei, were undermined by the Shu defeat at the Battle of Jieting. With the loss of Jieting (present-day Qin'an County, Gansu), Zhuge Liang's current location, Xicheng (西城; believed to be located 120 li southwest of present-day Tianshui, Gansu), became exposed and was in peril of being attacked by the Wei army. In the face of imminent danger, with the main Shu army deployed elsewhere and only a small group of soldiers in Xicheng, Zhuge Liang came up with a ploy to hold off the approaching enemy.

Zhuge Liang ordered all the gates to be opened and instructed soldiers disguised as civilians to sweep the roads while he sat on the viewing platform above the gates with two boys flanking him. He put on a calm and composed image by playing his guqin. When the Wei army led by Sima Yi arrived, Sima was surprised by the scene before him and he ordered a retreat after suspecting that there was an ambush inside the city. Zhuge Liang later explained that his strategy was a risky one. It worked because Zhuge Liang had a reputation for being a careful military tactician who hardly took risks, so Sima Yi came to the conclusion that there was an ambush upon seeing Zhuge's relaxed composure.

As a topic of academic study
Christopher Cotton, an economist from the University of Miami, and Chang Liu, a PhD Student in the Department of Finance at the Georgia Institute of Technology, used game theory to assess the Chinese military legends of Li Guang and his 100 horsemen (144 BC), and Zhuge Liang and the Empty City (228 AD). In the case of the military legends, the researchers found that bluffing arose naturally as the optimal strategy in each situation. The findings were published under the title 100 Horsemen and the empty city: A game theoretic examination of deception in Chinese military legend in the Journal of Peace Research in 2011.

Historicity
The basis for this story in Romance of the Three Kingdoms came from annotations to Zhuge Liang's biography in the historical text Sanguozhi, written by Chen Shou. The annotations came from San Shi (三事) written by Guo Chong (郭沖), and were added by Pei Songzhi to Zhuge Liang's biography. Guo Chong's account of the incident is as follows:

"Zhuge Liang garrisoned at Yangping (陽平; around present-day Hanzhong, Shaanxi) and ordered Wei Yan to lead the troops east. He left behind only 10,000 men to defend Yangping. Sima Yi led 200,000 troops to attack Zhuge Liang and he took a shortcut, bypassing Wei Yan's army and arriving at a place 60 li away from Zhuge Liang's location. Upon inspection, Sima Yi realised that Zhuge Liang's city was weakly defended. Zhuge Liang knew that Sima Yi was near, and he thought of recalling Wei Yan's army back to counter Sima Yi, but it was too late already, and his men were worried and terrified. Zhuge Liang remained calm and instructed his men to hide all flags and banners and silence the war drums. He then ordered all the gates to be opened and told his men to sweep and dust the ground. Sima Yi was under the impression that Zhuge Liang was cautious and prudent, and he was baffled by the sight before him and suspected that there was an ambush. He then withdrew his troops. The following day, Zhuge Liang clapped his hands, laughed, and told an aide that Sima Yi thought that there was an ambush and had retreated. Later, his scouts returned and reported that Sima Yi had indeed retreated. Sima Yi was very upset when he heard about it later."

After adding Guo Chong's account to Zhuge Liang's biography, Pei Songzhi made some personal comments about the account. He pointed out some fallacies in the account and dismissed it as fiction. He wrote:

"When Zhuge Liang garrisoned at Yangping, Sima Yi was serving as the Area Commander (都督) of Jing Province and he was stationed at Wancheng (宛城; present-day Wancheng District, Nanyang, Henan). He only came into confrontation with Zhuge Liang in Guanzhong after Cao Zhen's death (in 231). It was unlikely that the Wei government ordered Sima Yi to lead an army from Wancheng to attack Shu via Xicheng (西城) because there were heavy rains at that time (which would obstruct passage). There were no battles fought at Yangping before and after that period of time. Going by Guo Chong's account, if Sima Yi did lead 200,000 troops to attack Zhuge Liang, knew that Zhuge's position was weakly defended, and suspected that there was an ambush, he could have set up defences to resist Zhuge instead of retreating. According to Wei Yan's biography, which states: 'Each time Wei Yan followed Zhuge Liang to battle, he would request to command a separate detachment of about 10,000 men and take a different route and rendezvous with Zhuge's main force at Tong Pass (present-day Tongguan County, Shaanxi). Zhuge Liang rejected the plan, and Wei Yan felt that Zhuge was a coward and complained that his talent was not put to good use.' Zhuge Liang never agreed to allow Wei Yan to command a separate detachment of thousands of troops. If Guo Chong's account was true, how was it possible that Zhuge Liang would permit Wei Yan to lead a larger force ahead while he remained behind with a smaller army? Guo Chong's account was endorsed by the Prince of Fufeng (Sima Liang, a son of Sima Yi). However, the story puts Sima Yi in a negative light, and it does not make sense for a son to approve a story which demeans his father. When we read that 'the Prince of Fufeng generously endorsed Guo Chong's account', we know that this account is purely fiction."

Evidence from historical sources indicate that Sima Yi was indeed not present around the Jieting area (in present-day Gansu) at that time. The Battle of Jieting took place in 228 but Sima Yi's biography in the Book of Jin claimed that in 227, Sima Yi was stationed at Wancheng in the north of Jing Province. He led an army to suppress a rebellion by Meng Da at Xincheng (新城) in northwestern Jing Province, and returned to Wancheng after his victory. Later, he was summoned to the capital Luoyang to meet the Wei emperor Cao Rui, who consulted him on some affairs before ordering him to return to the garrison at Wancheng. Sima Yi only came into direct confrontation with Zhuge Liang after 230.

Yi Zhongtian, a professor from Xiamen University, commented on this incident in his book Pin San Guo (品三国). He pointed three fallacies in the story:
 * 1) Sima Yi did not dare to attack because he feared that there was an ambush inside the fortress. If so, he could have sent recces to scout ahead and check if there was really an ambush.
 * 2) Romance of the Three Kingdoms provided this description: "(Sima Yi) saw Zhuge Liang sitting at the top of the gates, smiling and playing his guqin and being oblivious to his surroundings." Based on this description, the distance between Sima Yi and Zhuge Liang must be very short, or else Sima would not be able to observe Zhuge's actions so clearly. If so, Sima Yi could have ordered an archer to kill Zhuge Liang (who was most likely within the archer's range, based on the description), which is in accordance with "defeat the enemy by capturing (or killing) their chief" (擒賊先擒王), the 18th of the Thirty-Six Stratagems.
 * 3) Both Guo Chong's account and Romance of the Three Kingdoms said that Sima Yi's army was superior to Zhuge Liang's in terms of size — Guo Chong's account stated that Sima Yi had 200,000 men while Zhuge Liang had 10,000 men; Romance of the Three Kingdoms mentioned that Sima Yi had 150,000 men while Zhuge Liang had only 2,500 men. If so, Sima Yi could have ordered his troops to surround Zhuge Liang's fortress first, and then wait for an opportunity to attack. [This is in accordance with Chapter Three of Sun Tzu's The Art of War, which states that "It is the rule in war, if our forces are ten to the enemy's one, to surround him..."]

Battle of Mikatagahara
In 1572, during the Sengoku Period in Japan, Tokugawa Ieyasu used the tactic during his retreat in the Battle of Mikatagahara. He commanded that the fortress gates remain open, and that braziers be lit to guide his retreating army back to safety. One officer beat a large war drum, seeking to add encouragement to the returning men of a noble, courageous retreat. When the enemy forces, led by Baba Nobuharu and Yamagata Masakage heard the drums, and saw the braziers and open gates, they assumed that Tokugawa was planning a trap, and so they stopped and made camp for the night.

Cultural references

 * In Chinese culture, the act of leaving one's house doors unlocked is sometimes called "setting up an empty fort strategy" (擺空城計).


 * The Portal Three Kingdoms expansion set of the Magic: The Gathering collectible card game include a card named "Empty City Ruse".