In all fighting, the direct method may be used for joining battle, but indirect methods will be needed in order to secure victory.
Indirect tactics, efficiently applied, are inexhaustible as Heaven and Earth, unending as the flow of rivers and streams; like the sun and moon, they end but to begin anew; like the four seasons, they pass away to return once more.
In battle, there are not more than two methods of attack–the direct and the indirect; yet these two in combination give rise to an endless series of maneuvers.
The onset of troops is like the rush of a torrent which will even roll stones along in its course.
The quality of decision is like the well-timed swoop of a falcon which enables it to strike and destroy its victim.
Therefore the good fighter will be terrible in his onset, and prompt in his decision.
Energy may be likened to the bending of a crossbow; decision, to the releasing of a trigger.
– The Art of War (5th century BC) – Sun Tzu