Complex sentence: The dual significance of the battle was that the Germans were unable to complete their occupation of the entire country, and the Yser area remained unoccupied, providing a propaganda coup to the Belgian forces on the Western Front for the duration of the war.
Split into simpler sentences: The dual significance of the battle was that the Germans were unable to complete their occupation of the entire country, and the Yser area remained unoccupied. The fact that providing a propaganda coup to the Belgian forces on the Western Front for the duration of the war.