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."
Ch`en Hao quotes the saying: "Birds and beasts when brought to bay will use their claws and teeth." Chang Yu says: "If your adversary has burned his boats and destroyed his cooking-pots, and is ready to stake all on the issue of a battle, he must not be pushed to extremities." Ho Shih illustrates the meaning by a story taken from the life of Yen-ch`ing. That general, together with his colleague Tu Chung-wei was surrounded by a vastly superior army of Khitans in the year 945 A.D. The country was bare and desert-like, and the little Chinese force was soon in dire straits for want of water. The wells they bored ran dry, and the men were reduced to squeezing lumps of mud and sucking out the moisture. Their ranks thinned rapidly, until at last Fu Yen-ch`ing exclaimed: "We are desperate men. Far better to die for our country than to go with fettered hands into captivity!" A strong gale happened to be blowing from the northeast and darkening the air with dense clouds of sandy dust. To Chung-wei was for waiting until this had abated before deciding on a final attack; but luckily another officer, Li Shou-cheng by name, was quicker to see an opportunity, and said: "They are many and we are few, but in the midst of this sandstorm our numbers will not be discernible; victory will go to the strenuous fighter, and the wind will be our best ally." Accordingly, Fu Yen-ch`ing made a sudden and wholly unexpected onslaught with his cavalry, routed the barbarians and succeeded in breaking through to safety.