In 1917, during the scene where Schofield travels through Écoust at night, there are white phosphorous flares being shot over the city, which helps to light his way.
It appeared that this village is still being held by Germans (albeit not many of them) who are now effectively behind enemy lines, as the Central powers had retreated, and the Allied powers were advancing. However, they were not under attack at the time, and clearly weren't expecting to be, as many of the soldiers were asleep or drinking.
Who was shooting the flares over the city? And what was the purpose of shooting so many of them? Was it some sort of signal, or was it to light the city in the darkness?