In the final years of the Qin Dynasty, the court's oppressive taxes and forced labor pushed the people to their breaking point, sparking widespread rebellions, the largest of which was led by Chen Sheng and Wu Guang. In 209 BC, their rebel army established the Zhang Chu regime and proclaimed Chen Sheng as king.
Soon after, Chen Wang appointed his friend Wu Chen as general, with Zhang Er and Chen Yu serving as left and right commanders, to march north and conquer the former territory of the Yue state. The people of Zhao, long discontented with Qin rule, eagerly cooperated when the rebel army arrived. Within a short time, Wu Chen captured over thirty cities, and his forces grew increasingly formidable.
After deliberation, Zhang Er and Chen Yu decided to support Wu Chen as the King of Zhao, with Zhang Er serving as Right Chancellor and Chen Yu as Grand General. Wu Chen then dispatched his general Han Guang to continue the northern campaign and seize the former territory of the Yan state.
Unexpectedly, as soon as Han Guang arrived in Yan territory, he declared himself King of Yan. Enraged upon learning this, Wu Chen personally led an army with Zhang Er and Chen Yu to attack Yan. But no sooner had they entered Yan lands than Wu Chen had a sudden whim: hiding his plan from Zhang and Chen, he took only a few personal guards deep into Yan territory to scout. However, the Yan defenses were tight—Wu Chen and most of his escort were captured, with only two guards escaping back.
After Han Guang imprisoned the King of Zhao, he demanded half of Zhao's territory in exchange for the king's release. Zhang Er and Chen Yu, desperate to negotiate, sent over a dozen envoys to the Yan camp—but each was executed by the Yan general, leaving no one willing to risk the mission.
Just as they were at their wit's end, one of Wu Chen's attendants volunteered to go. Zhang Er and Chen Yu immediately sent him. Upon meeting the Yan general, the attendant asked, "May I ask, General, do you know what kind of men Zhang Er and Chen Yu are?"
"I know, both of them are renowned sages. But now, they are both counting on me to release the King of Zhao," the Yan general replied.
The guard laughed heartily and said, "You're mistaken. When those two first joined Wu Chen, they captured dozens of cities with little effort—do you think they didn't want to claim kingship themselves? No, they merely deferred because Zhao was newly established and the people's loyalty uncertain. Now that Zhao is stable, they both secretly wish to split the land and rule as kings. So on the surface, they demand Wu Chen's release, but in truth, they'd be delighted if you killed him. If a single Zhao already looks down on Yan, imagine two Zhao states, with two wise kings joining forces to call you to account—then Yan's doom would be sealed."
The Yan general, completely fooled by the guard's words, hastily reported to Han Guang, who also believed the story and immediately released Wu Chen.
Later, people used the idiom "Left Hand, Right Hand" to describe mutual support.
Source: *Records of the Grand Historian*, "Biographies of Zhang Er and Chen Yu"
Meaning of the Idiom: Later, the Chinese idiom "左提右挈" came to describe how mutual support.