The spot where we intend to fight must not be made known; for then the enemy will have to prepare against a possible attack at several different points; and his forces being thus distributed in many directions, the numbers we shall have to face at any given point will be proportionately few.
It is essential to seek out enemy agents who have come to conduct espionage against you and to bribe them to serve you. Give them instructions and care for them. Thus doubled agents are recruited and used.
Be extremely subtle, even to the point of formlessness. Be extremely mysterious, even to the point of soundlessness. Thereby you can be the director of the opponent's fate.
Hence the saying: If you know the enemy and you know yourself, your victory will not stand in doubt; if you now Heaven and you know Earth, you may make your victory complete.
When the leader is morally weak and his discipline not strict, when his instructions and guidance are not enlightened, when there are no consistent rules, neighboring rulers will take advantage of this.