From Wikipedia

Line of fire
Battle of the Somme

 The plan for the Somme offensive evolved out of Allied strategic discussions at Chantilly, Oise in December 1915. Chaired by General Joseph Joffre, the commander-in-chief of the French Army, Allied representatives agreed on a concerted offensive against the Central Powers in 1916 by the French, British, Italian and Russian armies. The Somme offensive was to be the Anglo-French contribution to this general offensive, and was intended to create a rupture in the German line which could then be exploited with a decisive blow. With the German attack on Verdun on the River Meuse in February 1916, the Allies were forced to adapt their plans. The British Army took the lead on the Somme, though the French contribution remained significant.