Complex sentence: The front held by the French extended from Boesinghe to the north of Nordschoote, the ground to the north being a morass created by the Belgians, when they flooded the area during the Battle of the Yser in 1914.
Split into simpler sentences: The front held by the French extended from Boesinghe to the north of Nordschoote. The ground to the north was a morass created by the Belgians, when they flooded the area during the Battle of the Yser in 1914.