With the Germans on the high ground, overlooking the British troops, little effort was made or was indeed possible, to hide the intent to launch an offensive on the Somme. However, General Rawlinson made a plan for his Fourth Army based on a number of assumptions. Firstly, that the inexperienced New Army divisions, who made up the bulk of his Army, were only capable of executing a limited number of detailed instructions. This led to a simplistic, deliberate and inflexible plan. Rawlinson realised that this was the case but justified his approach in a Fourth Army Tactical Note:
'We must remember that owing to a large expansion of our army and the heavy casualties in experienced officers, the officers and troops generally do not now possess that military knowledge arising from a long and high state of training which enables them to react instinctively and promptly on sound lines in unexpected situations. They have become accustomed to deliberate action based on detailed orders.'
Rawlinson's second assumption was that the British artillery, now generously supplied with ammunition, would cut the enemy's wire, destroy the enemy trenches and their defenders up to 2,000 yards behind the German frontline. The plan was for a five day preliminary bombardment of the German first and second defensive systems by 1,537 guns. However, only a small proportion were of the heavier calibres.
The British infantry assault on a 24,000 yard frontage was to be launched in conjunction with the French on the southern flank. The first day's objectives included large portions of the German second position, which on the left flank required the infantry to take objectives such as the Pozieres Ridge. Here, 1st Dorset's objective was 4,000 yards behind the enemy frontline. However, in the southern sectors, the objectives were less deep, with 7th Division (8th and 9th Devon) having objectives beyond Mametz to a depth of 1,200 yards.
Most of the Fourth Army's five infantry corps were to attack with two divisions, with a third division prepared to take trench systems in depth and exploit success. For the following days, a series of plans had been made for the capture of the remainder of the second and the whole of the embryonic third defensive system and the formation of defensive flanks to hold open 'the breakthrough'.
Having broken the German lines, it was then planned that the infantry and four cavalry divisions of Gough's Reserve Army would launch themselves into the gap and plunge into the enemy's rear. Though planners were keen to exploit any success that presented itself, their general wish was to exploit northwards to outflank the Germans facing the BEF around Arras. This it was hoped would unhinge the entire German front and lead to a resumption of open warfare, where Haig's much prized, but largely inactive, Army reserve of five cavalry divisions could be employed.
Zero-Hour was set for 0730 hours on 'Z' Day. The British would have preferred to attack almost three hours earlier at dawn but the French insisted on a later time in order to ensure the accuracy of the final hours of intense artillery bombardment. The result was that, while the Allied gunners had a clear view of their targets, the German machine gunners could also clearly see the advancing British infantry.
Leaving their trenches, Fourth Army's Tactical Notes required infantry to:
'… push forward at a steady pace in successive lines, each line adding impetus to the preceding line. Although a steady pace for assaulting troops is recommended, occasions may arise where the rapid advance of some lightly-equipped men on some particular part of the enemy's defences may turn the scale.'
The first sentence of this paragraph is often used as evidence of simplicity and overconfidence in the power of the bombardment. The reality, however, was that if the lightly equipped men, sent forward to take important objectives, became casualties the heavily laden succeeding waves would be easy prey for any German machine guns that survived the bombardment.