At Chantilly last December the Allied military leaders agreed to launch coordinated summer offensives in an attempt to overwhelm the Central Powers. The British and French then decided to launch a joint offensive in July at the junction of their armies in the Somme valley. Originally the French were to make the main efforts here, with the British in a supporting role. But the demands of the Verdun bloodbath mean the French keep having to reduce their commitment. Joffre was originally offering some 39 divisions for the offensive, but by the start of the month he had to reduce that to 20, to the British commitment of somewhere between 25 and 30 divisions.
Now, with the renewed German offensive at Verdun, Joffre tells the British that he can spare just 12 divisions for the Somme offensive. This means that the character of the battle is transformed. The British are no longer playing second fiddle to the French: they will be the ones making the main effort.