Qi Zi, a senior official from the state of Qin stationed in Zheng, secretly sent word to Duke Mu of Qin, urging him to launch a surprise attack while Qi Zi controlled the Beixia Gate. Duke Mu, seeing a golden opportunity, ignored the warnings of his advisor Jian Shu and immediately dispatched three generals—Meng Ming, Xi Qi Shu, and Bai Yi Bing—to lead a distant campaign against Zheng. Jian Shu's own son was among the troops. At the farewell, Jian Shu embraced his son and wept bitterly, crying, "You will surely be ambushed by the Jin army at Xiao Pass, and I will come to collect your bones." When Duke Mu heard this, he furiously cursed Jian Shu, declaring him worthy of death.
When the Qin army, after a long march, arrived at the small state of Hua near Zheng, the Zheng merchant Xian Gao happened to be passing through on his way to trade in the Zhou capital. Realizing the Qin forces intended to attack his homeland, Xian Gao kept his composure and pretended to be an envoy sent by Duke Mu of Zheng. Approaching the Qin commander, he declared, 'Our ruler, knowing of your arrival, has sent me with livestock to reward your troops.' Having thus stalled the enemy, Xian Gao secretly dispatched a messenger to race back to Zheng with the urgent news of the impending invasion.
Duke Mu of Zheng received Mi Gao's secret report and immediately sent someone to check on Qi Zi and his men, finding them indeed with packed luggage, sharpening weapons and feeding their horses, ready to act as Qin's inside agents. After confirming Mi Gao's intelligence, Duke Mu dispatched Huang Wuzi to Qi Zi with this message: "We deeply regret not having properly entertained you. Now that your general Meng Ming is on his way, you may go with him."
Seeing that their plot had been exposed, the men from Qi fled separately to the states of Qi and Song.
Meng Ming received the news and realized the surprise attack could not succeed. He said dejectedly, "Zheng is already prepared. We have no internal allies. There is no hope of conquering Zheng. We might as well return." So he ordered the army to withdraw back to their country.
On their retreat, the Qin army passed through the treacherous Xiao Mountains, where they were ambushed by Jin forces. The entire Qin army was annihilated, and the three commanders, including Mengming, were captured alive by the Jin state.
Later, the idiom "sharpening weapons and feeding horses" came to describe preparing for battle, and also broadly refers to making thorough preparations in advance.
Source: *Zuo Zhuan*, Chapter "Duke Xi's 33rd Year"
Meaning of the Idiom: Later, the Chinese idiom "厉兵秣马" came to describe preparing for battle, and also broadly refers to making thorough preparations in advance.