to launch a surprise attack when the opponent is unprepared; to catch an enemy unawares

To take advantage of an opponent when they are off guard or have not made preparations.

(idiom) extremely well-equipped or fully prepared; nothing is lacking; everything is ready

To strike or act when others are unprepared; to take someone by surprise.

To take advantage of a moment when others are unprepared or off guard.

to take advantage of someone in an unguarded moment; to catch someone off guard

To catch an enemy off guard with a surprise attack; an idiom from Sunzi's "The Art of War" [[孙子兵法|孫子兵法|sun1 zi3 bing1 fa3]]