As a result of the failed endeavours in France in 1915, and in the spirit of greater cooperation enunciated by the new French premier Aristide Briand, the planning for the 1916 campaign in the west began with a conference. First the politicians met in Calais on 5 December, and then the military between the 6th and the 8th at Joffre's headquarters in Chantilly. The decision they all reached, a decision modified by the German offensive at Verdun that began on 21 February 1916, proclaimed greater unity of purpose. Coordinated allied attacks on all fronts, Western, Eastern and Italian, would eliminate the German advantage of interior lines. The Franco-British portion of those coordinated attacks became the Battle of the Somme.

This battle is significant for the relationship for two reasons. It marked the end of France's domination of the military alliance; and it was the only joint battle of the war. In 1915 British forces had been too small to play any independent role. French casualties incurred in 1914 and 1915, and increased by the effort to defend Verdun, left the British as the main military power of the Entente in the West. On the Somme, however, virtually equal numbers of French and British fought alongside each other in a single campaign. In 1917 the Allies would fight separately, apart from the French First Army that fought under Haig's orders in Flanders. In 1918 allied troops were intermingled on the battlefield.