Guo Jia, styled Fengxiao, hailed from Yangzhai in Yingchuan (present-day Yuxian, Henan). A master strategist, he earned Cao Cao's deep trust and high regard.
After Cao Cao defeated Yuan Shao, who controlled the four provinces of Ji, Qing, You, and Bing, he killed Yuan Shao's eldest son, Yuan Tan. Yuan Shao's other two sons, Yuan Shang and Yuan Xi, fled to the Wuhuan tribe in the Liao River region, seeking refuge under their leader, Chief Tadun. Taking advantage of the chaos, Tadun raided Han border territories, disrupting the lives and livelihoods of the local people. Cao Cao wanted to launch a campaign against Yuan Shang and Tadun, but some officials worried that if he marched far away, Liu Biao in Jingzhou might take the opportunity to send Liu Bei to attack Cao Cao's rear.
Guo Jia analyzed the situation and advised Cao Cao, "Your might now shakes the empire, but the Wuhuan tribes, relying on their remote location, will surely be unprepared. A surprise attack can wipe them out. If we delay, Yuan Shang and Yuan Xi will catch their breath, rally their remnants, the Wuhuan will respond, and Tadun will grow ambitious—then Ji and Qing provinces may slip from our grasp. Liu Biao is a mere talker; knowing his own talents fall short of Liu Bei's, he won't give Liu Bei real power, and Liu Bei, unappreciated, won't exert himself for Liu Biao. So rest assured in your campaign against the Wuhuan—there will be no trouble from the rear."
Cao Cao then led his army on campaign. After arriving at Yi County (in present-day Hebei), his strategist Guo Jia advised, "Speed is the essence of warfare. Now we are marching a thousand li to battle, but heavy supplies slow our pace. If the Wuhuan learn of our movements, they will prepare defenses. Better to leave the bulky equipment behind, have the troops travel light, and advance at double speed. By striking before the enemy expects us, we will achieve a decisive victory."
Following Guo Jia's strategy, Cao Cao's army marched swiftly, directly reaching the camp of Chief Tadun. The Wuhuan people, caught completely off guard, were utterly defeated. Tadun was killed in battle, while Yuan Shang and Yuan Xi fled to Liaodong, where they were later killed by the local governor, Sun Kang.
Later, the idiom "Speed is the soul of warfare" came to describe swift military action that catches the enemy off guard and achieves victory.
Source: *Records of the Three Kingdoms*, "Book of Wei: Biography of Guo Jia"
Meaning of the Idiom: Later, the Chinese idiom "兵贵神速" came to describe how swift military action that catches the enemy off guard and achieves victory.