China was historically divided into three kingdoms: Wei in the north, Shu in the southwest, and Wu in the southeast.
Cao Cao of Wei once led a 200,000-strong army south to conquer Wu and Shu. In response, Wu and Shu formed an alliance. To cross the Yangtze River, Cao Cao ordered his ships to be chained together, forming a floating bridge.
Zhou Yu, commander of the allied forces, devised a plan to attack with fire after careful analysis. However, he realized his strategy depended on a strong east wind, which failed to arrive for days. Worried, Zhou Yu received a note from Zhuge Liang, Shu's military adviser:
To fight Cao Cao
Fire will help you win
Everything is ready
Except the east wind
Zhou Yu urgently sought Zhuge Liang's help, who assured him the east wind would come within days. Two days later, the wind rose, enabling Zhou Yu to execute his plan and secure victory for the alliance.
This story later gave rise to the idiom "Everything is ready except what is crucial."