It is the rule in war, if our forces are ten to the enemy’s one, to
surround him; if five to one, to attack him;
Straightaway, without waiting for any further advantage.
if twice as numerous, to divide our army into two.
Note that 之 does not refer to the enemy, as in the two preceding
clauses. This sudden change of object is quite common in Chinese.
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: 以二敵一則一術爲正一術爲奇
“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.” [For
explanation of 正 and 奇, see V. 3, note.] Chang Yü 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.”