During the Warring States period, King Kuai of Yan appointed his prime minister, Zi Zhi, who was close friends with the wandering persuader Su Dai. When Su Dai gained favor in Qi and was sent as an envoy to Yan, King Kuai asked him, "How does Qi compare to Yan?" Su Dai replied, "Qi's ruler is no match for you, my king." Intrigued, King Kuai pressed, "What makes you say that?" Su Dai explained, "Because Qi's ruler does not trust his ministers as you do, my king." This flattery delighted King Kuai, who then entrusted even more power to Zi Zhi, ultimately leading to chaos in the state.
Su Dai said to the King of Yan, "In ancient times, wise rulers yielded their kingdoms to the virtuous. Zi Zhi is a virtuous man of our time. If Your Majesty yields the state to him, you will be remembered through the ages, just like the sage King Yao of old."
The King of Yan fell for the trick and actually handed over the reins of government to Zizhi
The King of Qi summoned his ministers and said, "The northern state of Yan is in great chaos. Heaven has given Qi the chance to expand our territory and increase our wealth. I have decided to raise an army and march north to seize this opportunity. I hope all of you will contribute your strength and wisdom to help me succeed."
Both civil and military officials urged the king to attack, with generals advocating for conquering cities, seizing wealth and people, while civil ministers pushed for military force to subdue Yan, install a puppet ruler, and have the new Yan king honor the Qi king as emperor.
Mencius also supported sending troops to Yan, but he argued that the goal was not to covet neighboring lands or wealth, nor to have Yan offer King Xuan of Qi an honorary title. Mencius said, "The world has been in turmoil for over five hundred years, and the people long for a sage ruler. Heaven has given you, Your Majesty, a chance to become that sage. Seeking kingship or emperorship is about personal gain, but a sage ruler rescues the people from suffering. If you send troops to help a neighbor quell its internal chaos, taking no land or treasure, Qi will set an example among the feudal lords by promoting benevolence and righteousness. If all states follow suit, the governance of the Three Sovereigns and Five Emperors will be restored in China."
King Xuan of Qi had long coveted the treasures of his neighboring state, so when Mencius advised restraint, the king ignored him. Qi's army invaded Yan, sweeping through like a knife through butter—but their looting and slaughter soon united the Yan people in fierce resistance. The Yan forces counterattacked, and Qi suffered a crushing defeat.
After Mencius saw his political advice repeatedly rejected, he resigned from his post and left the state of Qi.
Mencius was frustrated when his plan to promote benevolence and righteousness to pacify the world failed in Qi, and he couldn't help feeling upset. His disciple Chong Yu said, "Master, you seem quite unhappy. Before, I heard you teach everyone: 'A gentleman does not blame heaven, nor does he resent others.' What's wrong now?"
Mencius replied, "That was then, this is now. A few days ago I may have been displeased, but now I realize: within every five hundred years, a true king of benevolence must arise, and with him, helpers to guide the age. Since the founding of the Zhou Dynasty, over seven hundred years have passed—far beyond that cycle. Now is the time for action. Unless Heaven itself wishes to abandon the world, who else but me could take on this great task? Why should I still be unhappy?"
"That was then, this is now" also written as "this is now, that was then," describes how circumstances change as times differ.
Source: *Mencius*, Chapter "Gongsun Chou"
Meaning of the Idiom: Later, the Chinese idiom "彼一时，此一时" came to describe how circumstances change as times differ.