You capture couple of enemies, and use them as a workers, yea slaves you may say. Or your group capture enemies, and use them as a workers, or force them to bring explosives to their group or they will be shot in the back (they usually get shot by their old friends, because explosive they carry).
They had no rights, almost everybody could kill them or torture them, sometimes they would be swapped (exchanged) for prisoners on other side or ransom, or something else, favors etc.
All kind of slavery was there in that time, yes sexual too. I can not say it was normal because it was not, but it was usual.