纸上谈兵

In the Warring States Period, the State of Zhao had a famous general called Zhao She, whose son, Zhao Kuo, was very fond of reading books on military science and discussing strategy. He could recite military texts by heart, and when discussing warfare he spoke so clearly and logically that it seemed that even his father was not his match. When the State of Qin attacked the State of Zhao, the ruler of Zhao ordered Zhao Kuo to lead 400,000 men to resist（抵抗） the attack. But since Zhao Kuo had no practical experience of battle, he was defeated and lost his life.

Later people used this idiom to describe those who are good only at theorizing, and lack practical experience.