警戒部队的英文：
guarding troops; outpost troops; troops on alert

参考例句：

The commander called in the second battalion's outposts.
指挥官下令第二营警戒部队撤回。

guarding是什么意思：
adj. 保护的，监护的
v. guard的现在分词形式；保护

guarding position
防守位置

a guarded answer
谨慎的回答

...the Irish Guards
...爱尔兰近卫团

troops是什么意思：
n. troop的名词复数；部队

motorized troops
摩托化的部队

Daylight will reveal the disposition of their troops.
天亮之后，他们军队的部署就要显露出来。

concentrate troops at a strategic point
集中兵力于战略重点

outpost是什么意思：
n. 前哨,前哨部队,前哨地点

The commander called in the second battalion's outposts.
指挥官下令第二营警戒部队撤回。

The army and the people guard the island outpost together.
军民共守前哨岛屿。

The invading army overran a few scattered guerrilla outposts.
入侵军占领了游击队若干分散的前哨基地。