17. For should the enemy strengthen his van, he will weaken his rear; should he strengthen his rear, he will weaken his van; should he strengthen his left, he will weaken his right; should he strengthen his right, he will weaken his left. If he sends reinforcements everywhere, he will everywhere be weak.
In Frederick the Great’s Instructions to his Generals we read: “A defensive war is apt to betray us into too frequent detachment. Those generals who have had but little experience attempt to protect every point, while those who are better acquainted with their profession, having only the capital object in view, guard against a decisive blow, and acquiesce in smaller misfortunes to avoid greater.”
18. Numerical weakness comes from having to prepare against possible attacks; numerical strength, from compelling our adversary to make these preparations against us.
The highest generalship, in Col. Henderson’s words, is “to compel the enemy to disperse his army, and then to concentrate superior force against each fraction in turn.”
19. Knowing the place and the time of the coming battle, we may concentrate from the greatest distances in order to fight.
There is nothing about “defeating” anybody in this sentence, as Capt. Calthrop translates. What Sun Tzŭ evidently has in mind is that nice calculation of distances and that masterly employment of strategy which enable a general to divide his army for the purpose of a long and rapid march, and afterwards to effect a junction at precisely the right spot and the right hour in order to confront the enemy in overwhelming strength. Among many such successful junctions which military history records, one of the most dramatic and decisive was the appearance of Blücher just at the critical moment on the field of Waterloo.