攻其无备 (攻其無備) gōng qí wú bèi

gōng qí wú bèi set phrase attack [the enemy] when he is not prepared
Domain: Idiom 成语
Notes: For example, in the Warring States period text 孫子兵法 “Art of War” by Sunzi, section 1 (reworded from tr. by Giles 1910, p. 7; Guoyu '攻其無備'; Wikipedia '攻其無備')