坚壁清野 — “To strengthen fortifications and clear the fields of all resources”

Towards the end of the Eastern Han Dynasty, after Cao Cao suppressed the Yellow Turban Rebellion and occupied Yanzhou, he ambitiously prepared to seize the strategic area of Xuzhou. At that time, there was a very talented man named Xun Yu from Yingchuan Yingyang (present-day Xuchang, Henan). To avoid the chaos caused by Dong Zhuo, he moved to Jizhou and was treated as a distinguished guest by Yuan Shao. He, however, perceived that Yuan Shao would not accomplish great things and thus defected to Cao Cao. Cao Cao was overjoyed and appointed him Sima. From then on, Xun Yu followed Cao Cao in various campaigns, offering strategies and gaining Cao Cao's deep trust.

In 194 AD, Tao Qian, the governor of Xuzhou, died of illness and handed Xuzhou over to Liu Bei before his death. Upon hearing the news, Cao Cao's desire to capture Xuzhou became irresistible, and he eagerly prepared to send troops.

Xun Yu learned of Cao Cao's intention and advised him, 'In ancient times, Emperor Gaozu of Han secured Guanzhong, and Emperor Guangwu of Han controlled Henei. They both had a solid base of operations, which enabled them to advance and defeat enemies, and retreat to hold their ground, thus achieving great success. Now, if you disregard Yanzhou to attack Xuzhou, if we leave too many troops in Yanzhou, it will not be enough to take Xuzhou; if we leave too few, should Lü Bu take advantage of the emptiness, it will not be enough to defend Yanzhou. In the end, you will surely lose Yanzhou and fail to capture Xuzhou.'

He further pointed out, 'It is now the wheat harvest season. I hear that Xuzhou has already organized manpower to quickly cut down and transport the wheat outside the city into the city. This indicates they are prepared. Once rumors spread, they will certainly strengthen their defenses and transfer all their supplies, ready to confront us. If your army truly goes there, you will not be able to capture the city, nor will you obtain any provisions. Within ten days, your army will disintegrate without a fight.'

Cao Cao listened to Xun Yu's words and greatly admired his wisdom. From then on, he concentrated his forces and soon defeated Lü Bu. Later, he also defeated Liu Bei and successfully occupied Xuzhou.