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.
Set across the western front between 1914 and 1918, Verdun aims for a realistic-ish take on trench warfare with realistic ballistics, gas attacks, gas masks, artillery strikes, carefully-researched equipment, and awful death. While it does support deathmatch, the core team-based mode is about the attacks, defences, and counter-attacks of trench warfare.
Along with personal persistent progression, it has a neat-sounding squad progression where playing together with the same folks in a certain role improves you all.
Verdun's come a long way overs its many years of development, as this trailer showing the path from earliest art in 2006, across engine changes, updates, and so on shows. Future plans include more maps, more squad types, new gore, and more.
As for how the game is now, here, have a launch trailer: