After Emperor Guangwu of the Eastern Han Dynasty, Liu Xiu, ascended the throne, he generously rewarded his meritorious officials, but one named Peng Chong was overlooked. Peng Chong, originally the Governor of Yuyang (present-day Miyun, Hebei), had once sent cavalry to help Liu Xiu quell the rebellion of Wang Lang. After Liu Xiu declared himself emperor, others received promotions, yet Peng Chong remained in his old post, leaving him deeply resentful.
Peng Chong, the governor of Yuyang, served under Zhu Fu, the inspector of Youzhou. Zhu Fu, a hot-tempered man, demanded grain and taxes from Yuyang. Puffed up with pride over his own achievements, Peng Chong refused to send them and even voiced bitter complaints. Enraged, Zhu Fu fired off a scathing letter berating him. This only deepened Peng Chong's resentment toward Zhu Fu.
Soon after, Zhu Fu secretly reported to Emperor Guangwu that Peng Chong was acting suspiciously and might rebel. The emperor summoned Peng Chong to the capital. Peng Chong, suspecting Zhu Fu's scheming, demanded that Zhu Fu accompany him to court, but Emperor Guangwu refused. This deepened Peng Chong's suspicion and fear.
Peng Chong's wife, who habitually meddled in her husband's affairs, urged him not to go to the capital and instead declare independence. Heeding her advice, Peng Chong refused the imperial summons and launched an attack with over twenty thousand troops against Zhu Fu.
Zhu Fu heard that Peng Chong was raising an army to attack him, so he wrote a letter to him. The letter said: "Now the country has just achieved peace. Civil and military officials, regardless of their abilities, are all contributing to the nation. Yet you are recklessly abandoning yourself. You should understand that in all matters, one must not cause grief to one's loved ones and delight to one's enemies."
Peng Chong ignored Zhu Fu's warning and launched an attack on Ji City (southwest of modern Beijing), where Zhu Fu was stationed. When Emperor Guangwu learned of this, he quickly sent reinforcements to aid Zhu Fu. However, Peng Chong repelled the relief forces and trapped Zhu Fu inside Ji City. It was only with great difficulty that Zhu Fu later managed to escape.
After seizing Ji City, Peng Chong proclaimed himself King of Yan, swelling with pride and arrogance for a time, but in the end, his rebellion crumbled into utter defeat.
Later, people condensed "grieved by allies, delighted by enemies" into the idiom "Ally's Grief, Foe's Delight," used to describe actions that sadden one's own side while pleasing opponents.
Source: *Book of the Later Han*, "Biography of Zhu Fu"
Meaning of the Idiom: Later, the Chinese idiom "亲痛仇快" came to describe actions that sadden one's own side while pleasing opponents.