The British, the French, the Australian, The American, The Germans, the Dutch, The Austrian, The New Zealanders, The Italian, The Russian, The Indian, The African, The Turks, and the many other countries.
About this creation
I saw the actual photo on a re-enacting site, and only thinking hard I realised that the photos was of a dugout in a trench. I request you don't go searching for it if you can't stomach zombies.
And a picture of the trench and dugout. The round 1x1 bricks are meant to be corrugated iron. They look better IRL.
Quoting Mickey Bloom
Very good MOC, What country is the soldier supposed to be from?
If I'm right, then it's 1916, America didn't come in till 1917, the deceased was wearing a brodie, so either English or Commonwealth (Canada, Australia, New Zealand, (I don't think Indians wore brodies, but that's debatable, and other countries.)