The nature of war is constant change.

When the soldiers stand leaning on their spears, they are faint from want of food.

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.

If the enemy opens the door, you must race in.

Agitate him and ascertain the pattern of his movement.

You can ensure the success of your attacks if you only attack places that are undefended. You can ensure the safety of your defense if you only hold positions that cannot be attacked. Therefore, that general is skillful in attack whose opponent does not know what to defend; and he is skillful in defense whose opponent does not know what to attack.

When the enemy is relaxed, make them toil. When full, starve them. When settled, make them move.