Is there a word in English can express this?

Oristar, the fragments of a defeated army would most often be referred to as stragglers--and the act of "rounding them up" would be known as rallying those troops.

I should add, though, that the most common use of the verb "to straggle" in a military sense is to fall behind on a march, or to fall behind to avoid combat. In the specific instance you mention, however, stragglers would be appropriate.

Hi Setanta, straggler seems to be used more commonly referring to a soldier who has not engaged in a battle. But what I was looking for is an idiom for the soldiers who have fought with their enemy but failed to win the battle, who routed after a battle...

PS.
Thanks Clary for recommending some good dics. I've just read your replies yesterday.