Or: it's staged. Get a few soldiers into a ruined wood, throw some smoke, take a few pictures.

I'm sure you've seen this footage before:

[MEDIA=youtube]vaE6SH-_snE[/MEDIA]

This gets shown in pretty much every documentary about the First World War, and usually as authentic combat footage. But it's staged, too, to the best of my knowledge. The soldier who slides back down the trench side isn't shot, IIRC he just slipped and played dead.