When the Imperial German Army began the Battle of Verdun on the Meuse on 21 February 1916, French commanders diverted many of the divisions intended for the Somme and the "supporting" attack by the British became the principal effort.

Haig was not formally subordinate to Marshal Joseph Joffre but the British played a lesser role on the Western Front and complied with French strategy.

[7] By 31 May, the ambitious Franco-British plan for a decisive victory had been reduced to a limited offensive to relieve pressure on the French at Verdun and inflict attrition on the German armies in the west.

[8] The Chief of the German General Staff, Erich von Falkenhayn, intended to end the war by splitting the Anglo-French Entente in 1916, before its material superiority became unbeatable.

Falkenhayn planned to defeat the large number of reserves which the Entente could move into the path of a breakthrough, by threatening a sensitive point close to the existing front line and provoking the French into counter-attacking German positions.

[10] The unexpected length of the Verdun offensive, and the need to replace many drained units at Verdun, depleted the German strategic reserve placed behind the 6th Army, which held the Western Front from Hannescamps, 18 km (11 mi) south-west of Arras to St Eloi, south of Ypres and reduced the German counter-offensive strategy north of the Somme to one of passive and unyielding defence.

Later in the year, the Franco-British were able to attack on the Somme and at Verdun sequentially and the French recovered much of the ground lost on the east bank of the Meuse in October and December.

Falkenhayn, and then Hindenburg and Ludendorff, were forced to send divisions to Russia throughout the summer to prevent a collapse of the Austro-Hungarian army and then to conduct a counter-offensive against Romania, which declared war against the Central Powers on 27 August.

Rapid expansion created many vacancies for senior commands and specialist functions, which led to many appointments of retired officers and inexperienced newcomers.

Falkenhayn implied after the war that the psychology of German soldiers, shortage of manpower and lack of reserves made the policy inescapable, as the troops necessary to seal off breakthroughs did not exist.

Against Joffre's wishes, Haig abandoned the offensive north of the road, to reinforce the success in the south, where the Anglo-French forces pressed forward towards the German second line, preparatory to a general attack on 14 July.

[26] The Fourth Army attacked the German second defensive position from the Somme past Guillemont and Ginchy, north-west along the crest of the ridge to Pozières on the Albert–Bapaume road.

Most of the objective was captured and the German defence south of the Albert–Bapaume road put under great strain but the attack was not followed up due to British communication failures, casualties and disorganisation.

[27] The Battle of Fromelles was a subsidiary attack to support the Fourth Army on the Somme 80 km (50 mi) to the south, to exploit any weakening of the German defences opposite.

After the Battle of Albert the offensive had evolved to the capture of fortified villages, woods, and other terrain that offered observation for artillery fire, jumping-off points for more attacks, and other tactical advantages.

German defences ringed the British salient at Delville Wood to the north and had observation over the French Sixth Army area to the south towards the Somme river.

British attacks from Leuze Wood northwards to Ginchy had begun on 3 September, when the 7th Division captured the village and was then forced out by a German counter-attack.

The strategic objective of a breakthrough was not achieved but the tactical gains were considerable, the front line being advanced by 2,500–3,500 yards (2,300–3,200 m) and many casualties were inflicted on the German defenders.

Thiepval Ridge was well fortified and the German defenders fought with great determination, while the British co-ordination of infantry and artillery declined after the first day, due to confused fighting in the maze of trenches, dug-outs and shell-craters.

Organisational difficulties and deteriorating weather frustrated Joffre's intention to proceed by vigorous co-ordinated attacks by the Anglo-French armies, which became disjointed and declined in effectiveness during late September, at the same time as a revival occurred in the German defence.

The British experimented with new techniques in gas warfare, machine-gun bombardment and tank–infantry co-operation, as the Germans struggled to withstand the preponderance of men and material fielded by the Anglo-French, despite reorganisation and substantial reinforcements of troops, artillery and aircraft from Verdun.

The Reserve Army attacked to complete the capture of Regina Trench/Stuff Trench, north of Courcelette to the west end of Bazentin Ridge around Schwaben and Stuff Redoubts, during which bad weather caused great hardship and delay.

The Marine Brigade from Flanders and fresh German divisions brought from quiet fronts counter-attacked frequently and the British objectives were not secured until 11 November.

Political calculation, concern for Allied morale and Joffre's pressure for a continuation of attacks in France, to prevent German troop transfers to Russia and Italy also influenced Haig.

[42] After the Battle of the Ancre (13–18 November 1916), British attacks on the Somme front were stopped by the weather and military operations by both sides were mostly restricted to survival in the rain, snow, fog, mud fields, waterlogged trenches and shell-holes.

[46] Defensive positions held by the German army on the Somme after November 1916 were in poor condition; the garrisons were exhausted and censors of correspondence reported tiredness and low morale in front-line soldiers.

The German defence of the Ancre began to collapse under British attacks, which on 28 January 1917 caused Rupprecht to urge that the retirement to the Siegfriedstellung (Hindenburg Line) begin.

[52][53] British survivors of the battle had gained experience and the BEF learned how to conduct the mass industrial warfare which the continental armies had been fighting since 1914.

Experience of crossing the beaten zone showed that such lines or metalled roads could not be built quickly enough to sustain an advance, and that pausing while communications caught up allowed the defenders to recover.

[84] On 1 July 2016, at 7:28 a.m. British Summer Time, the UK observed a two minute silence to mark the start of the battle which began 100 years earlier.