It’s a British artilleryman in Salonika, 1915.  
As you can see, he’s just pulled the lanyard to fire his gun (the gun is in full recoil and the lanyard is blurred because it’s still whipping),
and at the exact same moment he’s been shot by a sniper…
you can see the puff of dust on the guy’s back as the bullet exits.  
Of course this means the sniper knew what he was doing.
He waited until the cannon was fired to shoot the guy,
since the boom would mask the crack of his shot.  Pretty crazy.
Thanks Tom for sending this in.