围魏救赵

wéi wèi jiù zhào

lit. to besiege 魏[Wei4] and rescue 趙|赵[Zhao4] (idiom)

fig. to relieve a besieged ally by attacking the home base of the besiegers

wéi

to encircle

wèi

name of a vassal state of the Zhou dynasty from 661 BC in Shanxi, one of the Seven Hero Warring States

jiù

to save

zhào

one of the seven states during the Warring States period (476–220 BC)