The Somme offensive began in July 1916.
The idea was to destroy German trenches with seven days of intensive bombardment.
British troops were told they would be able stroll through the shattered German lines and bring the war to an end.
But German troops had survived the shelling in bunkers 10m (32ft) underground.
When the British troops went "over the top" towards German lines they walked straight into an inferno of machine gunfire and thousands were killed.
Andrew Marr's Making Of Modern Britain continues on Wednesday, 11 October, 2009 at 2100 BST on BBC Two. Or watch afterwards on BBC
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