Lu Teng lived during the Northern and Southern Dynasties. His father-in-law was the Prince of Donglai of Northern Wei. Once, Emperor Xiaowu visited his father-in-law's home and, after speaking with Lu Teng, said to the Prince of Donglai, "You have gained a fine son-in-law."
After the Northern Wei split into Eastern and Western Wei, Lu Teng served as the Eastern Wei's General of the Western Expedition and Governor of Yangcheng. When the Western Wei attacked and captured Yangcheng, Lu Teng was taken prisoner. The Western Wei's prime minister, Yuwen Tai, recognizing his talent, personally untied his bonds and treated him with great courtesy. Feeling he had found a true patron, Lu Teng surrendered and was appointed General of Chariots and Cavalry by Yuwen Tai.
A few years later, Western Wei peasants Li Guangsi and Li Wu led an uprising. Yuwen Tai ordered Lu Teng to suppress it. Before departing, Yuwen Tai asked him how he would achieve victory. Lu Teng replied, "I can only adapt to circumstances as they arise; I cannot say what strategy I will use now."
Yuwen Tai said, "This is your chance to become a pillar general; I hope you achieve immediate success." With that, he unfastened his golden belt and gave it to Lu Teng.
Lu Teng arrived at the city held by the rebels, secretly ordered the construction of many scaling ladders, and under the cover of night, led his troops in an assault, capturing the city before dawn and executing Li Guangsi, Li Wu, and others. For this achievement, he was appointed Pillar of the State General and enfeoffed as Duke of Shangyong Commandery.
The idiom "adapt tactics to circumstances" refers to formulating contingency plans based on the situation.
Source: *Book of Zhou*, "Biography of Lu Teng"
Meaning of the Idiom: Later, the Chinese idiom "临机制变" came to describe formulating contingency plans based on the situation.