There is no place where espionage is not possible.
If your enemy is superior, evade him
If it is to your advantage, make a forward move; if not, stay where you are.
All war is deception.
He who is not sage and wise, humane and just, cannot use secret agent.s. And he who is not delicate and subtle cannot get the truth out of them.
Of old the expert in battle would first make himself invincible and then wait for his enemy to expose his vulnerability.