There is no place where espionage is not possible.

When you do battle, even if you are winning, if you continue for a long time it will dull your forces and blunt you edge...If you keep your armies out in the field for a long time, your supplies will be insufficient. Transportation of provisions itself consumes 20 times the amount transported.

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

Weigh the situation, then move.

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.

Of old the expert in battle would first make himself invincible and then wait for his enemy to expose his vulnerability.

Should one ask: 'how do I cope with a well-ordered enemy host about to attack me?' I reply: seize something he cherishes and he will conform to your desires.