The troops had penetrated deep into enemy lines.

部队已经深入敌军防线。

《牛津词典》

The guerrilla units mysteriously appear and disappear in operation behind enemy lines, without the enemy's being able to do anything about them.

游击队在敌人后方神出鬼没地活动， 敌人无奈他何。

《新英汉大辞典》

This document will enable her to pass through the enemy lines unmolested.

这证件将使她能平安无事地通过敌人防线。

youdao

Lee turned his horse toward the enemy lines behind his army.

VOA: special.2009.12.17

Hundreds or thousands of men were put in long lines across the front of the enemy position.

VOA: special.2009.12.03

Soon, based in London, he had an operation with more than 200 agents behind enemy lines.

CENTERFORSECURITYPOLICY: Center for Security Policy | CIA Director William Casey: His Analysis And Plain Speech Help Him Lead The Agency to the Top

Maj Marcus Mudd, the commander of the BRF, said his troops would be dropping in behind enemy lines.

BBC: UK troops launch Operation Tor Shezada in Afghanistan

But the 7th Infantry Division pushed on -- probing enemy lines, fighting bunker by bunker, hill by hill.

WHITEHOUSE: President Obama Awards Medal of Honor to Korean War Heroes