During the Warring States period, Sun Bin of Qi and Pang Juan of Wei studied military strategy together under the sage Guiguzi. Later, Pang Juan became a general in Wei, but knowing Sun Bin's talent surpassed his own, he lured Sun Bin to Wei, had his feet cut off, and tattooed his face with ink to disgrace him forever.
Later, when an envoy from the State of Qi arrived in Wei, Sun Bin secretly met with him. Recognizing Sun Bin's extraordinary talent, the envoy quietly smuggled him back to Qi. King Wei of Qi greatly admired Sun Bin's abilities and appointed him as the military strategist of Qi.
In 341 BCE, the State of Wei allied with Zhao to attack Han, which urgently sought help from Qi. King Wei of Qi appointed Tian Ji as general and Sun Bin as military advisor, leading 50,000 troops to rescue Han. Tian Ji followed Sun Bin's strategy, marching directly on Wei's capital, Daliang, forcing Wei general Pang Juan to withdraw from Han.
When Pang Juan heard that the Qi army was attacking Wei, he hastily withdrew his troops from Han. By then, the Qi forces had already crossed the border into Wei. Sun Bin then said to Tian Ji, "The Wei army has always been known for its ferocity and looks down on our Qi soldiers. A skilled commander must guide events according to the situation—that is, follow the enemy's line of thought, steer it, and lure them into a trap."
Sun Bin ordered his Qi army to build campfires for 100,000 soldiers on their first day in Wei territory, then for 50,000 on the second, and for just 30,000 on the third, creating the illusion of mass desertion to deceive Pang Juan into pursuing with only a small elite force.
Pang Juan led his troops in pursuit of the Qi army, only to discover that their cooking stoves dwindled day by day. Falling for Sun Bin's ruse, he believed half the Qi forces had deserted, breeding arrogance. Leaving the infantry behind, he pressed forward with only his elite cavalry, racing at breakneck speed.
Sun Bin judged that Pang Juan would arrive at Maling by night, so he set an ambush there, stripped the bark from a large tree by the road, and wrote on it the eight characters: "Pang Juan dies beneath this tree." He ordered the ambushed soldiers to shoot arrows as soon as they saw a torch lit.
That night, Pang Juan indeed arrived at Maling. Seeing what appeared to be writing on a tree, he ordered torches lit to examine it. Instantly, the Qi army unleashed a volley of arrows, killing and wounding countless Wei soldiers. Struck by six arrows, his strategies exhausted and his army defeated, Pang Juan drew his sword and committed suicide.
Later, the idiom "guide according to the situation" came to describe steering things in the direction of their natural development.
Source: *Records of the Grand Historian*, "Biographies of Sun Tzu and Wu Qi"
Meaning of the Idiom: Later, the Chinese idiom "因势利导" came to describe steering things in the direction of their natural development.