From Wikipedia

Battle Of The Somme

The Allied war strategy for 1916 was largely formulated during a conference at Chantilly, held between 6 December and 8 December 1915. It was decided that for the next year, simultaneous offensives would be mounted by the Russians in the East, the Italians (who had by now joined the Entente) in the Alps and the Anglo-French on the Western Front, thereby assailing the Central Powers from all sides.

On 19 December 1915, General Sir Douglas Haig had replaced General Sir John French as Commander-in-Chief of the British Expeditionary Force (BEF). Haig favoured a British offensive in Flanders—it was close to BEF supply routes via the Channel ports and had a strategic goal of driving the Germans from the North Sea coast of Belgium, from which their U-boats were menacing Britain. However, although there was no formal arrangement, the British were as yet the junior partner on the Western Front and had to comply with French policy. In January 1916 the French commander, General Joseph Joffre, had agreed to the BEF making their main effort in Flanders, but after further discussions in February, the decision was reached to mount a combined offensive where the French and British armies met astride the Somme River in Picardy.

Plans for the joint offensive on the Somme had barely begun to take shape when the Germans initiated the Battle of Verdun on 21 February 1916. As the French committed themselves to defending Verdun, their capacity to carry out their role on the Somme disappeared, and the burden shifted more to the British. France would end up contributing three corps to the opening of the attack (the XX, I Colonial, and XXXV Corps of the 6th Army). As the Battle of Verdun dragged on, the aim of the Somme offensive changed from delivering a decisive blow against Germany, to relieving the pressure on the French army, as the balance of forces changed to 13 French and 20 British divisions at the Somme.

There was also disagreement between Haig and his senior local commander, General Sir Henry Rawlinson, General Officer Commanding the British Fourth Army; who favoured a "bite and hold" approach rather than Haig's "decisive battle" concept.

The original British regular army, six divisions strong at the start of the war, had been effectively wiped out by the battles of 1914 and 1915. The bulk of the army was now made up of volunteers of the Territorial Force and Lord Kitchener's New Army, which had begun forming in August 1914. The expansion demanded generals for the senior commands, so promotion came at a rapid pace and did not always reflect competence or ability. Haig started the war as the commanding officer of British I Corps, then was promoted to command the British First Army, and then the BEF; and army group comprised of sixty divisions split between five armies. Yet this vast increase in raw numbers also diluted the overall troop quality, and undermined the confidence commanders had in their men; this was especially true of Rawlinson.

By mid-1916 the Fokker Scourge was over, and the Royal Flying Corps (RFC) had achieved air supremacy over the Somme battlefield. On the Somme front the RFC fielded ten squadrons of 185 aircraft against 129 German machines. The British pursued a vigorous offensive policy enabling them to spot for artillery, via aircraft or tethered balloons, while denying the Germans the same ability. Not until September would introduction of new aircraft swing the balance back in favour of the German Air Service once again; British losses, a product of Trenchard's aggressively offensive posture to the exclusion of superior German Air Service mobility and weather (prevailing winds blew toward the Allied side), contributed.

The German Army, on the defence, held the high ground and were aware of the intended attack; they had been practically unmolested since October 1914, which had allowed the time needed to construct extensive trench lines and deep shellproof bunkers.

British infantry attack plan for 1 July. The only success came in the south at Mametz and Montauban and on the French sector.