4 sentences

1. 他化名潜入敌后执行任务。

   tā huà​míng qián​rù dí​hòu zhí​xíng rèn​wu.

   He went behind enemy lines to carry out a mission under an alias.

   Source: Dong Chinese Dictionary

2. 我军迅速向敌后进行穿插。

   wǒ jūn xùn​sù xiàng dí​hòu jìn​xíng chuān​chā.

   Our army quickly maneuvered into the enemy's rear.

   Source: Dong Chinese Dictionary

3. 他们在敌后开展游击战争。

   tā​men zài dí​hòu kāi​zhǎn yóu​jī zhàn​zhēng.

   They carried out guerrilla warfare behind enemy lines.

   Source: Dong Chinese Dictionary

4. 他在敌后陷入了孤立无援的境地。

   tā zài dí​hòu xiàn​rù le gū​lì​wú​yuán de jìng​dì.

   He found himself isolated and without help behind enemy lines.

   Source: Dong Chinese Dictionary