Novel by Zola set in the Franco-Prussian war. Project Gutenberg English language version here. My Christmas reading sorted. Wiki on it here. It starts thus:
In the middle of the broad, fertile plain that stretches away in the direction of the Rhine, a mile and a quarter from Mulhausen, the camp was pitched. In the fitful light of the overcast August day, beneath the lowering sky that was filled with heavy drifting clouds, the long lines of squat white shelter-tents seemed to cower closer to the ground, and the muskets, stacked at regular intervals along the regimental fronts, made little spots of brightness, while over all the sentries with loaded pieces kept watch and ward, motionless as statues, straining their eyes to pierce the purplish mists that lay on the horizon and showed where the mighty river ran.