Cruel lessons of war; DOCUMENTARY OF THE WEEK Almost 20,000 British soldiers died at the Somme but their sacrifice wasn't in vain COMMEMORATING THE SOMME SUNDAY, BBC1,5.15pm.

This drama-documentary marks the 90th anniversary of the bitterest struggle of World War One and begins with a brief overview of the conflict in the summer of 1916.

The French army was engaged in a war of attrition at Verdun, where German forces were subjecting their enemy to prolonged assaults.

Desperate for the Hun to be diverted, a plan was drawn up between the Allies for British forces to attack further along the front, at the Somme River.

Told through recollections from soldiers on both sides, and dramatic reconstructions, the programme follows the fortunes of a group of soldiers, the 2nd Salford Pals, led by captain Tweed, who took up a position south of the town of Thiepval.

In front of them lay a German trench occupied by corporal Hinkel and his troops.

The latter had been under constant bombardment by Allied shells for days and knew something big was afoot.

They were right - at 7.30 on the morning of July 1, the first wave of soldiers were sent 'over the top' at walking pace, certain they would meet no resistance after the bombing.

They were wrong, and the Salford Pals were decimated. Thus unfolded one of the worst days in British military history, as 19,240 men were killed and 37,000 wounded. But there is more to this story.

The Allies learned from their mistakes and let officers on the front lines change battle plans as required.

The Battle of the Somme may have been synonymous with carnage but it provided a turning point that would see the Allies win the war.