Getting people to fight by letting the force of momentum work is like rolling logs and rocks. Logs and rocks are still when in a secure place, but roll on an incline; they remain stationary if square, they roll if round. Therefore, when people are skillfully led into battle, the momentum is like that of round rocks rolling down a high mountain - this is force.
And therefore only the enlightened sovereign and the worthy general who are able to use the most intelligent people as agents are certain to achieve great things.
It is a doctrine of war not to assume the enemy will not come, but rather to rely on one's readiness to meet him; not to presume that he will not attack, but rather to make one's self invincible.
It is imperative to contest all factions for complete victory, so the army is not garrisoned and the profit can be total. This is the law of strategic siege.
The sovereign must have full knowledge of the activities of the five sorts of agents. This knowledge must come from the double agents, and therefore it is mandatory that they be treated with the utmost liberality.