The idea

In the northeast of France is a region called Flanders. The battle line ran north to south through Flanders, British and French to the west, Germans to the east.

The countryside of Flanders is very flat and the ground is soft and often waterlogged. It is good trench-digging country, but there is no other shelter from attack.

The French plan was to attack from the south, but to confuse the Germans there would be a British attack from farther north to act as a diversion. However, there was a problem. In an earlier battle for the city of Verdun the French has suffered so many casualties that, when the attack was launched it was the British who ended up being the main attacking force.