围城打援

wéichéng-dǎyuán

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