hurrying, retreating units, with a vast force pressing forward to your
destruction. A's entire force is deliberately imperilled in order to
achieve the success of the plan as a whole, and upon A's tenacity, as
will be seen in what follows, the success of that plan entirely
depends.

[Illustration: Sketch 24.]

Well, while A is thus retreating--say, from his old position at A_1 on
the foregoing diagram to such a position as A_2, with Black swarming
up to crush him--the other corners of the square, B, C, and D, receive
the order to "swing"--that is, to go forward inclining to the left or
the right according to the command given.

Mark clearly that, until the order is given, the general commanding
Black cannot possibly tell whether the "swing" will be directed to
the left or to the right. Either B will close up against A, C spread
out farther to the left, and D come in between A and C (which is a
"swing" to the left) as in Sketch 25, or C will close up against A, B
will spread well out to the right, and D come up between A and B, as
in Sketch 26 (which is a "swing" to the right).

[Illustration: Sketch 25.]

Until the "swing" actually begins, Black, the enemy, cannot possibly
tell whether it is his left-hand units (1 to 8) or his right-hand
units (9 to 16) which will be affected. One of the two ends of his
line will have to meet White's concentrated effort; the other will be
left out in the cold. Black cannot make dispositions on the one
hypothesis or on the other. Whichever he chose, White would, of