Yue Fei, a renowned general of the Southern Song Dynasty who fought against the Jin invaders, showed great ambition from a young age. He not only mastered martial arts and military strategy but also cultivated a deep sense of national pride and patriotism.
At that time, the Jurchen tribe in the north established the Jin Kingdom and repeatedly defeated the Liao Kingdom. The Northern Song rulers, eager to reclaim the Sixteen Prefectures of Yanyun previously ceded to Liao, sent envoys to negotiate with Jin, agreeing to launch a joint pincer attack on Liao.
In 1122 AD, Yue Fei enlisted in the army and quickly earned the recognition of the imperial commissioner Liu, who appointed him as a squad leader.
In the fifth month of that year, the Song army's vanguard clashed with the Liao forces at Langou Dian. Due to incompetent commanders and poorly trained soldiers, they suffered a crushing defeat. The fleeing troops, with no means to survive, turned to robbing local households and traveling merchants. The band operating in Liu's jurisdiction was led by Tao Jun and Jia Jinhe.
General Yue Fei was ordered to eliminate a bandit force with just 200 soldiers. He first sent 30 men disguised as merchants, then positioned 100 soldiers in ambush at the foot of a nearby mountain, while he himself led a few dozen cavalry to challenge the enemy camp directly.
The trap worked perfectly. Tao Jun and his men fell for it completely, robbing the disguised merchants and forcing them to join their band. Soon after, Yue Fei led his troops in a direct challenge. Tao Jun and Jia marched out to meet him. After a brief skirmish, Yue Fei feigned retreat, luring the enemy toward a nearby hill. At a single signal, hidden soldiers sprang from ambush, the disguised merchants turned their weapons against their captors, and Tao Jun and Jia were captured alive on the spot.
During the battles at Pingding Tao and Jia, Yue Fei demonstrated exceptional military skill. Later, he served under Deputy Commander Zong Ze. After the Jin army destroyed the Liao dynasty, they launched a massive invasion of the Song, and Yue Fei repeatedly led troops to repel them, earning Zong Ze's praise. Zong Ze told him, "Your talent and martial prowess rival even the greatest generals of antiquity. But you excel only at field battles—this is not a complete strategy. I have an ancient battle formation diagram; study it well, and your future will be boundless!"
Yue Fei accepted the battle map and tactfully said, "Thank you for your kindness, Marshal. However, I believe that fighting in formation is the norm in military strategy; the brilliance of its application lies in the mind. As a commander, I think the most important thing is to know oneself and the enemy, adapt to changing circumstances, and only then can one achieve victory through surprise."
Hearing this, Zong Ze could not help but show an approving expression, knowing that Yue Fei was destined for greatness.
Later, people used the idiom "The wonder of application lies in the mind" to describe extremely flexible and masterful tactics in battle.
Source: *History of Song*, "Biography of Yue Fei"
Meaning of the Idiom: Later, the Chinese idiom "运用之妙，存乎一心" came to describe how extremely flexible and masterful tactics in battle.