15. 故兵以詐立以利動
In war, practise dissimulation, and you will succeed.
According to Tu Mu, 立 stands for 立勝. Cf. [I. § 18]. In the tactics of Turenne, deception of the enemy, especially as to the numerical strength of his troops, took a very prominent position.[[172]]
Move only if there is a real advantage to be gained.
This is the interpretation of all the commentators except Wang Hsi, who has the brief note 誘之也 “Entice out the enemy” (by offering him some apparent advantage).
16. 以分合爲變者也
Whether to concentrate or to divide your troops, must be decided by circumstances.
17. 故其疾如風其徐如林
Let your rapidity be that of the wind,
The simile is doubly appropriate, because the wind is not only swift but, as Mei Yao-ch‘ên points out, 無形跡 “invisible and leaves no tracks.”