In 684 BC, the states of Qi and Lu went to war. Duke Zhuang of Lu prepared to lead his troops into battle. A man named Cao Gui came to see him and asked, "Your Majesty, on what basis do you plan to fight the Qi army?"
Duke Zhuang of Lu said, "I treat my people well—whenever I have fine clothes or delicacies, I always share them with others." Upon hearing this, Cao Gui replied, "Such small favors do not reach all the people, so they won't risk their lives fighting for you." The Duke added, "In my sacrifices to the gods, I report the offerings honestly, never exaggerating." Cao Gui responded, "That is merely minor sincerity; the gods cannot bless you to victory with that alone." After a moment's thought, the Duke said, "I handle all legal cases in the state with great care, striving to judge them fairly and reasonably." Cao Gui then said with delight, "Now that is truly doing good for the people. With this, you can fight a battle. Please allow me to join you in the campaign."
Duke Zhuang agreed, and so he and Cao Gui rode together in the same war chariot as the army set out. The two forces met at Changshao in the state of Lu.
When the Qi army beat their drums to advance, Duke Zhuang of Lu was about to order a counterattack, but Cao Gui stopped him twice. Only after the Qi drums had sounded for the third time did Cao Gui finally tell the duke, "Now is the time to strike."
When the Lu army's drums sounded, the soldiers charged forward immediately, fighting with such ferocity that they utterly defeated the Qi forces.
As Duke Zhuang of Lu was about to order a pursuit, his advisor Cao Gui stopped him. Only after dismounting to inspect the tracks of the Qi army's retreating chariots and scanning their chaotic rout did Cao Gui allow the duke to give the chase, driving the Qi forces out of Lu's borders.
After winning the battle, Duke Zhuang of Lu asked Cao Gui for the reasoning behind his command. Cao Gui replied:
Cao Gui explained, "War is won by spirit. At the first drumbeat, our soldiers' courage peaks; at the second, it wanes; at the third, it's spent. So we struck while our spirit was high and defeated the Qi army. Qi is a powerful state—I feared they might be feigning retreat. But after observing their chaotic chariot tracks and scattered banners, I knew they were truly beaten, and urged the duke to pursue, securing total victory."
Duke Zhuang of Lu listened and was thoroughly convinced.
Later, the idiom "One Drum to Build Momentum" came to be used as a metaphor for finishing a task in one go while the drive is strong.
Source: *Zuo Zhuan*, Chapter "The Tenth Year of Duke Zhuang"
Meaning of the Idiom: Later, the Chinese idiom "一鼓作气" came to describe finishing a task in one go while the drive is strong.