surround him; if five to one, to attack him;
[Straightway, without waiting for any further advantage.]
if twice as numerous, to divide our army into two.
[Tu Mu takes exception to the saying; and at first sight, indeed, it
appears to violate a fundamental principle of war. Ts’ao Kung, however,
gives a clue to Sun Tzŭ’s meaning: "Being two to the enemy’s one, we
may use one part of our army in the regular way, and the other for some
special diversion." Chang Yu thus further elucidates the point: "If our
force is twice as numerous as that of the enemy, it should be split up
into two divisions, one to meet the enemy in front, and one to fall
upon his rear; if he replies to the frontal attack, he may be crushed
from behind; if to the rearward attack, he may be crushed in front."
This is what is meant by saying that ‘one part may be used in the
regular way, and the other for some special diversion.’ Tu Mu does not
understand that dividing one’s army is simply an irregular, just as
concentrating it is the regular, strategical method, and he is too
hasty in calling this a mistake."]