A big army, fearful and hateful of elves, is standing in front of the forest they all (or vast majority of) reside in. Behind them is a stream, and beyond the forest is the coast. What is to stop them from setting fire to the side of the forest close to them and letting it spread?
To clarify (I know this is very late and should ideally have been edited yesterday) — I want to see if it makes sense as something that has happened, and see if it is perhaps impossible for some reason. This is something I would like to have happen and framed as "that terrible thing we did during the war". Magic is not versatile enough to help either side. Perhaps if they came, at the height of summer, from the coast side by day and set several fires there, then by night, after the wind turned, they set fires from the other side. I'm simply not sure how possible it is to burn so big a place. I don't need it to burn entirely, for my purposes it simply has to have been gruesome enough to be a harrowing memory.