covering force meaning in Chinese

掩护部队

As for the assault force , the containing force and the reserve force in a tactical operation , the first is for annihilating the enemy , the second for preserving oneself , and the third is for either purpose according to circumstances - - either for annihilating the enemy ( in which case it reinforces the assault force or serves as a pursuit force ) , or for self - preservation ( in which case it reinforces the containing force or serves as a covering force )
战术上的突击队、钳制队、预备队，第一种为了消灭敌人，第二种为了保存自己，第三种准备依情况使用于两个目的? ?或者增援突击队，或者作为追击队，都是为了消灭敌人；或者增援钳制队，或者作为掩护队，都是为了保存自己。