The Germans played major part in the cause of this battle. On 21st February 1916 they attacked the French at Verdun - about 150 miles to the south east. The Germans intended it to be overwhelming and hoped either for a breakthrough or to kill so many French soldiers so that their motivation to continue the War would be broken. In the end the breakthrough did not occur, and the French didn't break. Verdun developed into a battle of wearing away, and, by December, each side had suffered about 350,000 casualties.
By June, the French army was under heavy attack at Verdun, They hoped that this offensive on the Somme would force the Germans to withdraw troops from Verdun.
The Battle of the Somme was designed to relieve the pressure on the French suffering at Verdun.
Another cause for the offensive was the fact that that Allies believed that they could break through the German lines and end the war.
Haig planned this offensive as a breakthrough battle. He claimed that the lengthy artillery bombardment before the attack would entirely destroy the German trench system and kill all the defenders. He said that this would allow the British troops to simply walk into the empty positions.
By the summer of 1916, “Kitchener's Army” was fully equipped, trained, and organized. The Kitchener's Army all-volunteer army formed in the United Kingdom during the First World War. Haig wanted to put these troops into battle.
The Germans contained the attack with locally available reserves, so didn't withdraw any troops from Verdun, but the Allies pushed them back.
The Allies only captured only a narrow strip of land 20 miles long and, at its furthest penetration 7 miles deep.
Around 418,000 British soldiers and around 194,000 French soldiers were casualties of this battle.
It is estimated that German casualties were in the region of 500,000.
On the first day of the battle, The Allies suffered about 60,000 casualties (20,000 of them dead), and this made it the worst day in the history of the British army. It is uncertain whether the German army could have survived another Somme, the British and French armies certainly couldn't.
The 'Kitchener's Army' was shattered.
Lloyd George, the new British Prime Minister, lost total confidence in his army commander, Haig. The two were at odds for the remainder of the war, something that had an opposing effect on the British war effort.