围城打援wéi chéng dǎ yuán

一种作战方略。一部兵力包围并佯攻敌城，另设伏兵攻打歼灭闻讯前来解围的城外援敌。

英besiege a city to annihilate the enemy relief force; besiege the enemy in order to strike at his reinforcements;