On July 1, 1916, 13 divisions of Commonwealth forces launched an offensive on the German line, supported by a French attack to the south.
Despite an intense, seven-day artillery bombardment the German defences were barely touched, with concrete bunkers protecting the soldiers, so the attack met unexpectedly fierce and deadly resistance.
The Germans exploited their good defensive positions on higher ground when the British and French troops attacked at 7.30am.
The British suffered 58,000 casualties – a third of them killed – making it the worst day in the history of the Army.
In the following weeks huge resources of men and equipment were used in a bid to exploit the very modest successes of the first day.
The German Army resisted tenaciously and meant a major battle for every village or wood.
It was only at the end of September that one of the initial July 1 targets, Thiepval, was captured.
The battle ended in November.
Britain had suffered 420,000 casualties, the French 200,000 and the Germans 500,000.
Allied forces gained some land, but it reached only just over seven miles at its deepest points.