In history, it was. ... they found themselves hiding in a field of carrots. So, as a badge, they all pulled up carrots and stuck them on their helmets, so's they'd know who their friends were and incidentally have a nourishing snack for later, which is never to be sneezed at on a battlefield.
Terry Pratchett, Discworld, fantasy, humor, light satire ... If this sounds interesting, Night Watch is as good a place to start as any. It does not assume familiarity with the dozens of previous novels in the series. It's actually better than most because it is a prequel and has less of the baggage accreted over the ages.
For Discworld fans who missed this one, Sam Vimes gets sent back in time and meets himself in the wild days when he first joined the Watch. As usual, things go very wrong, but come out right in the end.