I’d imagine the smell of decay is taking creative liberties with the truth. Any damp forest will have a certain smell of decay to it, more likely due to the fungus and rotting wood, than the smell of rotting bodies. That said, I’m sure being in the area will amplify this, given the gravitas of the event. I visited Normandy a couple of years ago and seeing the beaches, the American cemetery, the bunkers... it was like a punch to the gut. I’d imagine going to Verdun or Somme has a similar effect, even if you’re only slightly aware of history.