The great generals of old first ensured that they themselves were beyond defeat and then waited for the enemy to make themselves vulnerable. Thus we can say that although you have it in your own hands to place yourself beyond defeat, you cannot, of yourself, bring about the defeat of the enemy. Whilst you are unsure of victory, defend; when you are sure of victory, attack. Defense should indicate that you are not in a position to defeat the enemy, attack that you are even stronger than you need to be. A skilled defender digs himself in deeper than the ninth level of the Earth; a skilled attacker falls on the enemy from above the ninth level of Heaven. In this way you can both protect yourself completely and ensure total victory.
~ Sun Tzu, The Art of War