Taken during the Battle of the Somme (July - Nov 1916) where close to 1,000,000 men were killed or wounded.
From the information on the reverse it says that the picture was taken from an aircraft but it was more likely taken from an observation balloon.
Allied troops can be seen leaving the trenches and advancing across "no mans land", in the upper left shells can be seen exploding in the German trenches and German reinforcements can be seen moving towards the line.
I would recommend clicking on the original size to see the detail