On the thirteenth day of the ninth month in the thirtieth year of Duke Xi of Lu's reign, Duke Wen of Jin and Duke Mu of Qin joined forces to besiege the state of Zheng. The Jin army camped at Hanling, while the Qin forces stationed themselves at Sinan, pressing Zheng from both sides, leaving the kingdom on the brink of collapse.
Facing a dire crisis, Duke Wen of Zheng heeded the advice of his minister Yizhi Hu and decided to send Zhu Zhiwu as an envoy to the state of Qin to break the alliance between Qin and Jin. However, Zhu Zhiwu, who had never been given an important post, seized the chance to air his grievances: "In my prime, I was no better than others; now that I am old, I can do even less." Duke Wen admitted his mistake in not promoting Zhu Zhiwu earlier but urged him with the shared interests of the state and its people. Putting the greater good first, Zhu Zhiwu agreed to the mission. Source: *Zuo Zhuan*, "The 30th Year of Duke Xi of Lu"
That night, Zhu Zhiwu was lowered down the city wall by rope, slipping out of the capital to meet Duke Mu of Qin. He said, "Qin and Jin are besieging Zheng, and Zheng knows it is doomed. If destroying Zheng benefits you, then your campaign is worthwhile. But crossing Jin to hold Zheng as your border territory—you know that would be difficult. Why then destroy Zheng only to strengthen your neighbor? Strengthening your neighbor weakens you. If you spare Zheng as a host on your eastern road, when Qin envoys travel and run short of supplies, Zheng can provide for them. This would do you no harm."
After Zhu Zhiwu's analysis, it was crystal clear who would benefit and who would suffer from the destruction of the state of Zheng—a revelation enough to shake Duke Mu of Qin's resolve to attack. But Zhu Zhiwu did not stop there. He pressed further, exposing the insatiable greed and treacherous nature of the state of Jin, and reminded Duke Mu, "Do not forget the bitter lessons of the past."
After Zhu Zhiwu's analysis, Duke Mu of Qin's mind cleared completely, and he secretly made a peace treaty with Zheng, sending his generals Qi Zi, Feng Sun, and Yang Sun to station troops in Zheng to assist in its defense, while he led his main army back to Qin.
Duke Wen of Jin, seeing that Qin had unilaterally withdrawn its troops and the hope of a joint attack on Zheng was lost, though he had suffered a "silent loss"—unable to voice his grievance—considered the timing not yet ripe to openly break with Qin, so he could only order his army to withdraw from Zheng.
"The idiom 'host from the east' originally referred to the host on an eastward journey. Later, it came to mean the host who entertains guests."
Source: *Zuo Zhuan*, "Duke Xi's Thirtieth Year"
Meaning of the Idiom: Later, the Chinese idiom "东道主" came to describe how the host who entertains guests.