A postcard from France

I discovered this post card in a box of old photographs belonging to my Grandparents. It is postmarked 'Field Post Office' and stamped 'Passed Field Censor' and dated 1917.
On the back my Grandpa has written to Granny saying that she would understand the significance of the card.  When we looked at the family tree we discovered that they were married on 13th of August 1913.
Grandpa served in the Army in France throughout the 1st World War, and fortunately came home safely.