What was the key to winning a great battle in ancient China? According to author Zhu Fengjia (朱逢甲), who wrote the Book of Spies (《间书》) in the Qing dynasty (1616 – 1911), it was espionage: “In ancient times, when a great military general confronted another of the same level, the one who used spies would win.”

Zhu based his assessment on a long list of successful spying tactics that swayed battles and even the fate of the nation in ancient China. For example, spies turned the tide when the Qin state fought the Zhao state at the Battle of Changping in 260 BCE. According to The Records of the Grand Historian (《史记》) by Sima Qian (司马迁), the states entered a two-year stalemate with Zhao defending a position along the Dan River south of Changping Pass (in present-day Shanxi province), led by brilliant military general Lian Po (廉颇).

Fan Ju (范雎), prime minister of Qin, sent spies into the Qin army to spread rumors among the ranks that Zhao was confident of besting Lian and his army, since they thought he was a poor general––but feared the younger general, Zhao Kuo (赵括). The King of Zhao took the bait, sacked Lian, and put Zhao Kuo in charge despite the young man’s lack of military experience. Zhao abandoned Lian’s defensive tactics and attacked with his main force across the river, only to be routed by the Qin. They quickly surrendered, and 400,000 of their soldiers were executed.

The very earliest record of espionage in China can be found in The Commentary of Zuo (《左传》), a narrative history written from the 8th to 5th centuries BCE. It records that deposed emperor Shao Kang of the Xia dynasty (c. 2070 – 1600 BCE) sent spies to investigate the son of his usurper, Han Zhuo (寒浞). Though no further details are supplied, it suggests espionage existed in China at least 3,000 years ago.

In The Art of War, from as early as the 5th century BCE, Sun Tzu devoted his final chapter to “Using Spies,” and stressed that “None in the whole army are more intimate relations to be maintained than with spies. None should be more liberally rewarded. In no other business should greater secrecy be preserved.”

During the Northern and Southern dynasties (420 – 589), Wei Xiaokuan (韦孝宽) from the Western Wei state used spies and a cunning letter to defeat his Eastern Wei enemy, General Niu Daoheng (牛道恒). Niu had come close to attacking Wei in the city of Yiyang (in present-day Hunan province), and Wei wanted to get rid of him. He had spies gather samples of Niu’s handwriting and then forged a letter saying Niu planned to defect to Western Wei. He even had the letter weathered, and finished as if in a hurry by singeing it slightly under a lamp. Spies delivered the letter to Niu’s superior Duan Chen (段琛), who then began to suspect Niu and began to undermine his command of the army. With Eastern Wei forces destabilized and lacking leadership from either Niu or Duan, Wei Xiaokuan launched an attack, won the battle, and captured them both.