During the Warring States period, the various feudal lords constantly fought to expand their territories, with the state of Qin being the most powerful.
In 260 BCE, the state of Qin launched a massive invasion of Zhao. The King of Zhao appointed Lian Po as general to lead the army against the Qin forces. Unexpectedly, the Zhao army suffered repeated defeats and even lost a commandant. Alarmed by Qin's strength, the king panicked and summoned Yu Qing and Lou Chang for counsel, saying, "I am determined to fight the Qin army to the death. What do you think?"
“It's useless. I think we should send an envoy to sue for peace with Qin,” Lou Chang shook his head helplessly.
Yu Qing countered, 'I advocate for a ceasefire, but not for suing for peace. This time, Qin has deployed hundreds of thousands of troops against Zhao, holding nothing back. Their goal is to crush Zhao completely, so they won't accept any peace talks. In my view, we should send envoys with lavish gifts to seek aid from Chu and Wei; if Qin believes Chu and Wei are joining forces with us against them, they will surely be intimidated. Only then can we negotiate peace successfully.'
King Zhao of Zhao rejected Yu Qing's advice and still sent an envoy to negotiate peace with the Qin army. The King of Qin received the envoy. When news arrived, King Zhao was delighted. But Yu Qing warned, "Since Qin sees that Zhao, Chu, and Wei have not united, they will surely press their attack."
As Yu Qing had predicted, the Qin army swept through like a bamboo split, surrounding the Zhao capital of Handan, leaving the kingdom on the brink of collapse.
Later, Qin, its forces exhausted, temporarily withdrew from Handan but demanded that Zhao cede six cities as a condition for peace. Upon hearing this, King of Zhao summoned his ministers to discuss the matter. Some advocated accepting Qin's terms, but Yu Qing countered, "Your Majesty, do you believe the Qin army withdrew because they were worn out from the prolonged siege, or out of genuine concern for Zhao?"
"The State of Qin attacked Zhao with all its might—how could it show any affection?" said the King of Zhao. "Exactly!" Yu Qing replied. "Since Qin is already exhausted from war, yet you would gift them six cities for nothing. That would only strengthen Qin, and they will attack Zhao again."
Impressed by Yu Qing's reasoning, the King of Zhao sent him to forge an alliance with Qi, Han, and Wei against Qin. As a result, Qin promptly dispatched an envoy to negotiate peace with Zhao.
Later, people used the idiom "sparing no effort" to mean putting all one's strength into something.
Source: *Records of the Grand Historian*, "Biographies of Lord Pingyuan and Yu Qing"
Meaning of the Idiom: Later, the Chinese idiom "不遗余力" came to describe putting all one's strength into something.