Li Ch`uan and Tu Mu, with extraordinary inability to see a metaphor, take these words quite literally of food and drink that have been poisoned by the enemy. Ch`en Hao and Chang Yu carefully point out that the saying has a wider application.
The commentators explain this rather singular piece of advice by saying that a man whose heart is set on returning home will fight to the death against any attempt to bar his way, and is therefore too dangerous an opponent to be tackled. Chang Yu quotes the words of Han Hsin: "Invincible is the soldier who hath his desire and returneth homewards." A marvelous tale is told of Ts`ao Ts`ao's courage and resource in ch. 1 of the SAN KUO CHI: In 198 A.D., he was besieging Chang Hsiu in Jang, when Liu Piao sent reinforcements with a view to cutting off Ts`ao's retreat. The latter was obliged to draw off his troops, only to find himself hemmed in between two enemies, who were guarding each outlet of a narrow pass in which he had engaged himself. In this desperate plight Ts`ao waited until nightfall, when he bored a tunnel into the mountain side and laid an ambush in it. As soon as the whole army had passed by, the hidden troops fell on his rear, while Ts`ao himself turned and met his pursuers in front, so that they were thrown into confusion and annihilated. Ts`ao Ts`ao said afterwards: "The brigands tried to check my army in its retreat and brought me to battle in a desperate position: hence I knew how to overcome them."