The expansion of their armies and armament made it at length practicable for the British to undertake, in accordance with the general Allied policy, an offensive campaign on a large scale in the summer of 1916. It was decided that the French should co-operate and that the thrust, should be made up the valley of the Somme. In his despatch of 23rd December 1916 the British Commander-in-Chief has defined the objects of the offensive as threefold: to relieve the pressure on Verdun, where the German assault, had been designed partly to frustrate the impending blow; to assist the Allies in the other theatres of war by stopping any further transfer of German troops from the Western front; and to wear down the strength of the enemy forces. The British would have preferred a somewhat later date in the summer, which would have permitted of an increase of men and munitions and a period of further training for the new levies, but the continually increasing strain at Verdun forced their hands. It was agreed therefore that the combined attack should be launched not later than the end of June. The British share in the joint operations was committed to the Fourth Army under General Rawlinson. Arrangements were also made for a subsidiary attack on the northern flank of the selected area by troops from General Allenby's Third Army, and a skeleton Reserve Army was formed in rear under General Gough.
Owing to their commanding situation and bare glacis, destitute of cover for assaulting infantry, the enemy positions which confronted the British on the watershed between the Somme and the rivers of South-Western Belgium were in themselves of immense strength and had been converted by unremitting and skilfully directed industry, and by every technical device known to modern military art, into fortifications as nearly impregnable as any in history. Against them, indeed, along a considerable part of the front, British valour was on 1st July to dash itself in vain, for not merely was the subsidiary operation abortive, but in the northern sector of the main blow, from Thiepval to Serre, the assaulting lines withered away under the deadly combination of artillery, machine guns and wire. At the end of the day the Commander-in-Chief made up his mind perforce to cut his losses and not persevere for the moment with the attack in this sector. The 2 northern Corps of the 5 which formed General Rawlinson's command were handed over to General Gough. The latter's augmented Army was directed to act as a pivot on which our advance on the right could swing, and for the meantime to confine itself to a steady and methodical pressure. In the southern half of the British area, however, the first defence system, including trench lines, redoubts, woods, and villages, for a frontage of over 6 miles to a depth of a mile or more, was in our possession by 15th July. Of the second system, which ran along the southern crest of the main ridge from Guillemont through Longueval and the two Bazentins to Pozières, some 3½ miles were captured in the middle of July. For the remainder there ensued a stern and prolonged struggle (the second phase of the battle), in which the British troops, not without being mauled in the process, satisfactorily fulfilled the main object of the offensive. As they strained forward, the role of the Reserve Army became one of more active co-operation. By the second week of September, not merely had the enemy's Second Line been won, but at certain points very considerable penetration had been effected beyond. "Practically the whole of the forward crest of the main ridge from Delville Wood to beyond Pozières was now in our hands."{40} Meanwhile on the right the French had carried their lines by a series of brilliantly conceived and vigorously executed operations to within striking distance of Péronne.
It remained now to develop in a third phase the advantages obtained at the cost of so much blood and labour. Between Morval and Le Sars, behind the last of the enemy's; original systems of defence which now faced the victorious British, there had been added since the summer several new lines on the north-eastern slopes of the main ridge. None were, however, of the elaborate nature of those already stormed. Though fresh Bavarian Divisions had been thrown in, incontestable evidence betrayed a weakening of the German morale and an acute strain on his reserves. His guns, though enormously increased, were outweighed by the British artillery; and much was hoped from our as yet untried weapon of the tank The failure at Verdun, the collapse in Italy, the intervention of Roumania, the opening of the offensive in the Balkans, all seemed to presage the waning of Germany's star. Towards the end of August capable military opinion believed that we were approaching a stage when bold and energetic action might yield great and possibly decisive results. Consequently, as General Rawlinson now told his Corps commanders, Sir Douglas Haig was going “all out,” with every possible resource in men and engines of war, to bring the Battle of the Somme to a successful and immediate conclusion.