We should take them by surprise and strike when they're unprepared.
Tips
history
From 《孙子·计篇》 ('Sun Tzu, chapter on Calculations'): 攻其无备，出其不意 ('attack where they are unprepared; appear where you are not expected'). The idiom is half of one of Sun Tzu's most-quoted lines on surprise.
usage
Often paired with 出其不意 (chūqíbùyì, 'take by surprise') as a fixed couplet - they reinforce the same idea of catching the opponent off guard.
