[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."] 36. When you surround an army, leave an outlet free. [This does not mean that the enemy is to be allowed to escape. The object, as Tu Mu puts it, is "to make him believe that there is a road to safety, and thus prevent his fighting with the courage of despair." Tu Mu adds pleasantly: "After that, you may crush him."] Do not press a desperate foe too hard.