More than one person has asked for the story behind the word troop. In short, the question comes down to this: if the plural troops is synonymous with the plural soldiers, is the singular troop the same as the singular soldier?

The answer is: not really, although you couldn't tell that by the headlines, which commonly feature such phrasing as "Three Troops Killed." (来源：英语博客 http://space.englishcn.com)

Troop is a linguistic oddity; it sometimes functions as a count noun and sometimes as a non-count noun. Like snow and money, troop is a non-count noun; it is often used attributively (like an adjective) before another noun (think "troop level," "troop count," or "troop movement," for example).  When used in this non-count sense, troop refers to a collection of people, usually a military unit. (来源：英语论坛 http://bbs.englishcn.com)

But of course, troops also acts as a count noun plural, meaning "soldiers," as in "three troops." What makes this even more complicated is that troop can be pluralized to mean "a number of units of soldiers," as when we hear "The army sent three troops of MPs to the border."

(来源：英语分类信息 http://fl.englishcn.com)

But troop is not generally considered a singular count noun and using it that way is not considered standard (except, perhaps, in headlines).

(来源：EnglishCN英语博客基地)