◎ To besiege the State of Wei to rescue the State of Zhao,meaning to relieve a besieged ally by attacking the besiegers.
In 354 BC,Pang Juan led an army to besiege Handan,the capital of Zhao. Zhao asked Qi for help and the King of Qi sent Tian Ji and Sun Bin to rescue it. Sun thought that as the main force of Wei's army was in Zhao,Wei should be in a weak defense,therefore,he led his troops to attack Daliang,the capital of Wei. Wei had no other choice but to withdraw its troops from Handan. On its way home,the troops were intercepted by Qi at Guiling Pass,and almost annihilated.(https://www.daowen.com)