Li Jing, a renowned general of the early Tang Dynasty, played a pivotal role in establishing the Li family's rule.
Shortly after Emperor Gaozu of Tang, Li Yuan, ascended the throne, the general Li Jing advised him to launch a campaign against Xiao Xian, who had declared himself emperor in the middle Yangtze region. Li Yuan accepted the proposal, appointing his nephew Li Xiaomu as commander-in-chief and Li Jing as deputy to lead the expedition.
In August 621 AD, Li Xiaomu and Li Jing led their army to Kuizhou (present-day Fengjie, Sichuan). Xiao Xian, believing the autumn floods made the Yangtze's Three Gorges impassable, assumed the Tang forces would never dare risk the treacherous waters, so he paid them no mind.
Many Tang generals believed attacking by water during the flood season was too risky, arguing they should wait until the waters receded to strike Xiao Xian's stronghold at Jiangling. But Li Jing countered, "Speed is paramount in war. Xiao Xian's defenses are lax precisely because he expects no attack during the floods. This is our golden opportunity—strike now, catch him off guard, and win a decisive victory."
Li Xiaomu followed Li Jing's advice and advanced on Yiling. Xiao Xian, upon hearing the news, sent his general Wen Shihong with tens of thousands of elite troops to station along the Qingjiang River to resist the Tang army. Li Xiaomu planned to attack, but Li Jing said:
"Wen Shihong is a fierce general under Xiao Xian, and his troops fight very bravely. They have just stationed at Qingjiang, their morale is high, and it will be difficult to defeat them for now. It's better to wait until their morale wanes before launching an attack."
Li Xiaomu ignored the advice and personally led his troops into battle, only to be soundly defeated by Wen Shihong. The enemy seized the opportunity to pursue, plundering the various supplies and equipment abandoned by the Tang army.
Li Jing led his troops to reinforce Li Xiaomu and, noticing the enemy soldiers were each burdened with heavy loads, seized the moment to launch a counterattack, decisively defeating the enemy and turning the tide of battle.
Li Xiaomu and Li Jing pressed their advantage, driving straight to the gates of Jiangling. When the two armies clashed, the Tang forces proved unstoppable, defeating several of Xiao Xian's top generals in quick succession. With no escape left, Xiao Xian had no choice but to open the city gates and surrender.
Later, the idiom "Opportunity Must Not Be Missed" came to describe a chance that should not be let slip.
Source: *Old Book of Tang*, "Biography of Li Jing"
Meaning of the Idiom: Later, the Chinese idiom "机不可失" came to describe a chance that should not be let slip.