During the late Eastern Han Dynasty, after suppressing the Yellow Turban Rebellion, Cao Cao occupied Yanzhou and then marched east to seize Xuzhou. But the local strongman Zhang Miao allied with the warlord Lü Bu, who attacked and took most of Yanzhou, including the key city of Puyang. Cao Cao urgently withdrew from Xuzhou to counterattack Lü Bu at Puyang. Lü Bu was fierce, and the stalemate dragged on; Cao Cao could not win quickly. Soon, Xuzhou's defender Tao Qian died of illness and ceded Xuzhou to Liu Bei. Cao Cao's desire to take Xuzhou grew even more urgent, planning to capture it first and then eliminate Lü Bu.
Cao Cao's strategist Xun Yu advised against a hasty attack on Xuzhou, warning that Lü Bu might strike while they were vulnerable. He said, "It's wheat harvest season, and reports say Xuzhou is rushing to cut and store the grain inside the city—they're preparing for war. Once the wheat is secured, they'll fortify defenses, evacuate the countryside, and move supplies. Our army will have no foothold. With their 'fortify the walls and clear the fields' strategy, we can't storm the city or plunder resources. Within ten days, our entire force would collapse without a fight."
After hearing Xun Yu's analysis, Cao Cao was deeply impressed and decided to abandon the plan to march east, focusing instead on confronting Lü Bu. This strategy paid off handsomely, as Cao Cao decisively defeated Lü Bu and pacified Yanzhou.
Later, "Strengthen Walls, Clear Fields" came to refer to a tactic of starving and trapping the enemy.
Source: *Records of the Three Kingdoms*, "Biography of Xun Yu"
Meaning of the Idiom: Later, the Chinese idiom "坚壁清野" came to describe a tactic of starving and trapping the enemy.