Agincourt, as this explains they would likely been overwhelmed and killed by the French, it was amazing that Henry and his troops prevailed, going on the numbers involved.
"In any event, Henry ordered the slaughter of what were perhaps several thousand French prisoners, sparing only the most high ranked (presumably those most likely to fetch a large ransom under the chivalric system of warfare). According to most chroniclers, Henry's fear was that the prisoners (who, in an unusual turn of events, actually outnumbered their captors) would realize their advantage in numbers, rearm themselves with the weapons strewn about the field and overwhelm the exhausted English forces. Though ruthless, Henry's decision was thus arguably justifiable given the situation of the battle; even the French chroniclers do not criticise him for it.[56] In his study of the battle, John Keegan [57] argued that the main aim was not to actually kill the French knights but rather to terrorise them into submission and quell any possibility they might resume the fight, which would probably have caused the uncommitted French reserve forces to join the fray as well.
there is obviously a lot more, but you can read it for yourself.