new historical sources, published on the Centenary of the Battle of the Somme, and on unpublished personal papers of his ancestor, the General de Castelnau, second in command of the French army in 1916, Benoît Chenu places these events in a global context, highlighting their military and diplomatic interactions. He reveals a major strategic error made by the French leaders during the first week of the Battle of the Somme, depriving the Allies of victory, which would undoubtedly have brought about the end of war that year. These revelations shed new light on the contribution of the British to this war, which has often been underestimated in history.