The First World War is a not a common setting for first-person shooters. This perhaps isn’t a surprise, given that guns were a bit rubbish then, most deaths came from being shelled while huddled in trenches, gas was a grinding terror, and tactics often didn’t seem much more advanced than “Run quickly towards the enemy while yelling and hope they don’t machine gun you all.” That said, I’ve heard good things about WWI FPS Verdun [official site].
After almost two years in various forms of open beta and early access, the tactical multiplayer FPS today officially launched. It’s on Steam for 15.29/19,35 right now.