攻其无备

Explanation: It: pronoun, refers to the enemy. When no preparedness to take advantage of the enemy attack. 　　Usage: And language style; as subject, attribute; for military 　　Source: Sun Tzu "Sun Tzu Posts count": "Attack is unprepared, surprise." Thesaurus:  do stb.when people are off guard, take sb. unawares, to help a joint cause Wish watch, Surprise, Ji lei Less Their ears, sudden; as sudden as lightening; sudden peal of thunder leaves no time for covering ears