纸上谈兵 — “To be an armchair strategist”

During the Warring States period, Zhao She, a great general of the Zhao state, once defeated the invading Qin army with fewer troops and was promoted to a high-ranking official by King Huiwen of Zhao. He had a son named Zhao Kuo, who had thoroughly studied military texts since childhood and loved to discuss military affairs. Others often couldn't out-argue him, making him very proud and believing himself invincible. However, Zhao She was very worried about him, believing he was merely 'talking about war on paper' (纸上谈兵), and said: 'If Zhao state ever appoints him as a general, he will surely lead the Zhao army to defeat.'

Indeed, in 259 BC, the Qin army invaded again, and the Zhao army resisted at Changping (near modern Gaoping County, Shanxi). By then, Zhao She had passed away. Lian Po was in charge of commanding the entire army. Though old, he was very skillful in warfare, preventing the Qin army from achieving victory. Knowing that a prolonged stalemate was disadvantageous, Qin employed a counter-intelligence scheme. They sent people to Zhao state to spread rumors, saying, 'The Qin army fears General Zhao Kuo, Zhao She's son, the most.' The King of Zhao was deceived and replaced Lian Po with Zhao Kuo.

Zhao Kuo, thinking he was very capable in battle, rigidly followed military texts. Upon arriving at Changping, he completely changed Lian Po's battle plan. As a result, over 400,000 Zhao soldiers were annihilated, and he himself was killed by Qin arrows.