以逸待劳 — “To conserve one's strength and wait at ease for an exhausted enemy”

During the late Western Han Dynasty, the warlord Wei Xiao from Long-Gan broke away from Emperor Liu Xiu and joined Gongsun Shu, who had proclaimed himself emperor in Sichuan. Enraged, Liu Xiu dispatched troops to attack Wei Xiao, but his forces were defeated. Liu Xiu then assigned Feng Yi, the General-in-Chief of the Western Expedition, to capture Gouyi.

Upon hearing this, Wei Xiao immediately ordered his subordinate, Xing Xun, to rush to Gouyi and seize advantageous terrain. When Feng Yi's officers learned of Xing Xun's large army, they advised against engaging in battle. However, Feng Yi declared resolutely, "We must seize Gouyi using the strategy of 'Yi Yi Dai Lao' (waiting at ease for the fatigued enemy)."

Feng Yi commanded his troops to march rapidly, reaching Gouyi before Xing Xun's forces and occupying it. He then strictly sealed off all news, closed the city gates, and had the drums and flags silenced, allowing his soldiers to rest and recuperate.

When Xing Xun's weary army finally arrived at the city walls, drums suddenly boomed from the city tower, and Feng Yi's commander's banner was unfurled. Caught entirely off guard, Xing Xun's troops were terrified and scattered in all directions. Feng Yi promptly opened the city gates, led his forces out, and decisively defeated the enemy.

The idiom "Yi Yi Dai Lao" refers to the strategy of allowing one's own army to rest and build up strength, waiting for the enemy, who has traveled a long distance and is exhausted, to arrive, thereby achieving the objective of defeating them.