In 208 AD, the warlord Cao Cao led his massive army southward, confronting the allied forces of Sun Quan and Liu Bei at the Red Cliffs.
At the time, Zhou Yu, the Grand Commander of Eastern Wu, was directing the defense on the southern bank of the Red Cliffs, with Zhuge Liang, Liu Bei's military strategist, advising on battle plans. Both agreed that only a fire attack could defeat Cao Cao.
But when everything was ready, Zhou Yu realized it was deep winter and the northwest wind blew daily—if he used fire attack, not only would he fail to burn Cao Cao's ships, but the flames would turn back on his own fleet. In his desperation, he spat blood and collapsed onto his sickbed.
Knowing the true cause of Zhou Yu's illness, Zhuge Liang visited him and deliberately asked why he had fallen sick. Zhou Yu, unwilling to reveal the truth, replied, "Man is subject to sudden calamities—who can guarantee against illness?" Seeing his evasion, Zhuge Liang smiled and said, "Heaven has unforeseen storms—who can predict them?" Catching the double meaning in Zhuge Liang's words, Zhou Yu asked if he had a cure. Zhuge Liang replied, "I have a prescription that will surely cure your ailment, Governor." He then took paper and brush and wrote sixteen characters: "To defeat Cao Cao, fire is the key; all is ready, only the east wind is missing."
Zhou Yu was deeply impressed and humbly asked Zhuge Liang for a strategy to defeat the enemy. Zhuge Liang, skilled in reading the skies, knew a southeast wind was due in a few days, but he mysteriously claimed, "I can summon the wind and rain—I will borrow a southeast wind for you." Hearing this, Zhou Yu's illness vanished instantly.
Zhuge Liang ordered Zhou Yu to build the Seven-Star Altar on Nanping Mountain, where he ascended and performed rituals, pretending to summon the wind and rain. Meanwhile, Zhou Yu prepared everything for the fire attack, waiting only for the southeast wind to rise before immediately advancing his troops.
Three nights later, in the dead of night, the southeast wind finally arrived. Zhou Yu immediately ordered an attack on Cao Cao's forces. He also sent men to Nanping Mountain to kill Zhuge Liang, but Zhuge Liang had already left Eastern Wu and returned to Jingzhou, where he deployed Liu Bei's army to launch their own assault on Cao Cao's troops.
The Battle of Red Cliffs ended with Eastern Wu's victory and Cao Cao's defeat.
Later, the idiom "All is ready except the east wind" came to mean everything is prepared except the one crucial condition.
Source: *Romance of the Three Kingdoms*
Meaning of the Idiom: Later, the Chinese idiom "万事俱备，只欠东风" came to describe everything is prepared except the one crucial condition.