1 July 1916. The British have fired a million and a half shells and tell the Pals battalions, volunteers from the same street, the same mill, the same football club, that they can walk across. The wire is not cut. The machine guns have not been destroyed. By nightfall: 57,000 British casualties, 19,000 of them dead.
Why then
Agreed in December 1915 as a joint Anglo-French blow, the offensive changes once Verdun starts eating the French army in February. It becomes a mostly British attack meant to take pressure off France, built on industrial output, railway timetables and a theory of wearing the enemy down. A week of bombardment does not do what the staff promised.
What they wanted
Haig hopes to break the German line. Berlin means to hold it cheaply with dug-in machine guns and artillery. The Pals were told the guns had done the job.
What changed
They keep going until November. The line moves about six miles. The worst day in British Army history becomes the template for two more years of the Western Front.
Still argued
Revisionists argue the Somme taught the British army the methods that won in 1918, and wore the German army down more than the gains on the ground suggest; critics answer that the lessons cost far too much.