Pierre des Vallieres was a French cavalry officer who had been both a staff officer and a field commander before being appointed to head the French Military Mission to the British Army in December 1915. His position at GHQ gave him a unique insight into the preparations for the 1916 Battle of the Somme and the 1917 Battle of Arras. The reports on these events that he sent to French headquarters, and his private diary entries, are here translated into English for the first time. His frequent and often acerbic comments on British failures to follow French advice, together with informative reports from other liaison officers on the British performance, provide a rare glimpse of the British Expeditionary Force in France from the outside.