During the late Eastern Han Dynasty, warlords carved up the land. Liu Zhang, the governor of Yizhou, was an incompetent ruler. His officials Fa Zheng and Zhang Song believed he could not hold Yizhou in this era of fierce competition, so they plotted to bring in Liu Bei, the governor of Jingzhou, whom they considered reliable, but lacked the opportunity to do so.
Soon after, Cao Cao planned to attack Zhang Lu in Hanzhong. Liu Zhang realized that if Hanzhong fell, Yizhou would be indefensible. Heeding Zhang Song's advice, Liu Zhang sent Fa Zheng to Jingzhou to invite Liu Bei into Shu to capture Hanzhong.
When Fa Zheng arrived in Jingzhou, he met with Liu Bei and secretly advised him, "Seize this chance to take Shu." Though Liu Bei coveted Shu, he hesitated, saying, "Liu Zhang and I are both of the Han imperial clan—I cannot turn our family into enemies." He agreed only to enter Shu and help Liu Zhang defend against invaders, refusing to fight his own kin.
After Liu Zhang died, his son surrendered to Cao Cao, and only then did Liu Bei adopt Fa Zheng's advice, launching an attack to seize Yizhou and establish the Shu Kingdom.
Liu Bei, recognizing Fa Zheng's contributions, appointed him as the governor of Shu Commandery.
After Fazheng became governor, the entire capital fell under his jurisdiction, and he used his power to settle personal scores: he repaid every past kindness and avenged every past grievance without exception, even sentencing some to death.
Some people, unhappy with Fa Zheng's arrogance, reported to Chancellor Zhuge Liang, urging him to inform Liu Bei. Zhuge Liang shook his head and replied, "When our lord was stationed in Gong'an, he feared Cao Cao's might to the north and faced Sun Quan's threat to the east. Even Lady Sun, so close at hand, posed a constant worry. It was Fa Zheng who urged him to march into Yizhou, leading to our success today. Our lord trusts him and appointed him governor of Shu Commandery. As long as Fa Zheng remains loyal in principle, our lord will not curb his power."
Those who had grievances against Fa Zheng fell silent and withdrew upon hearing this.
Later, the idiom "Trouble at One's Elbow" came to describe an incident occurring in close proximity.
Source: *Records of the Three Kingdoms*, "Biography of Fa Zheng"
Meaning of the Idiom: Later, the Chinese idiom "变生肘腋" came to describe an incident occurring in close proximity.