Other forms: harried; harrying; harries
When one army sends raiding parties into another's territory, they're harrying them. They're not making an all-out attack, they're just trying to bother and distract the other army.
Although harry is not a word you hear commonly now, it does frequently occur as harried — which is an adjective used to describe what it feels like to be asked for things from all sides. You might feel harried during final exams, or two days before Christmas, if you haven't yet started your shopping.