To Catch Something, First Let It Go
Cornered prey will often mount a final desperate attack. To prevent this you let the enemy believe he still has a chance for freedom. His will to fight is thus dampened by his desire to escape. When in the end the freedom is proven a falsehood the enemy's morale will be defeated and he will surrender without a fight.
Six Dynasties Period China
During the Southern Song period, general Tan Dao-Ji launched an attack against the north on behalf of the emperor. Throughout the campaign he seized cities and destroyed fortifications, taking more than four thousand prisoners. His advisors suggested that he should execute them all and erect a victory mound with the dead. Tan Dao-Ji replied: "At this time we have attacked the guilty and consoled the people. The army of a true king takes the upright as its position, so why is it necessary to slay the people?" He released all the prisoners and sent them back to their homes. These former prisoners told their kinsmen of their capture and release and of the fair treatment they received at the hands of general Tan. Thereupon the barbarians dwelling in the region were elated, and wherever general Tan, went a great many came forward to give their allegiance to the emperor.