Art of War 9.21

Ts‘ao Kung explains this as “felling trees to clear a passage,” and Chang Yü says: “Every army sends out scouts to climb high places and observe the enemy. If a scout sees that the trees of a forest are moving and shaking, he may know that they are being cut down to clear a passage for the enemy’s march.”

Whenever the meaning of a passage happens to be somewhat elusive, Capt. Calthrop seems to consider himself justified in giving free rein to the imagination. Thus, though his text is here identical with ours, he renders the above: “Broken branches and trodden grass, as of the passing of a large host, must be regarded with suspicion.” Tu Yu’s explanation, borrowed from Ts‘ao Kung, is as follows: “The presence of a number of screens or sheds in the midst of thick vegetation is a sure sign that the enemy has fled and, fearing pursuit, has constructed these hiding-places in order to make us suspect an ambush.” It appears that these “screens” were hastily knotted together out of any long grass which the retreating enemy happened to come across.