There is no place where espionage is not possible.
Ground which can be abandoned but is hard to re-occupy is called entangling.
Sweat more during peace: bleed less during war
In the practical art of war, the best thing of all is to take the enemy's country whole and intact; to shatter and destroy it is not so good.
Pretend inferiority and encourage his arrogance.
If I wish to engage, then the enemy, for all his high ramparts and deep moat, cannot avoid engagement; I attack that which he is obliged to rescue.