By sunset on July 1st, 1916, the British Army had suffered over 57,000 casualties, the bloodiest single day in its history. The Battle of the Somme would grind on for 141 more days, pull three million men into its orbit, and leave more than a million casualties for a prize measured in miles of shattered mud. Yet out of that catastrophe came the birth of modern combined-arms warfare, including the first tanks ever deployed on a battlefield.

This episode looks past the monolithic myth of senseless slaughter to examine the mechanisms: how Verdun hijacked the original French-led plan, why Kitchener's volunteer Pals battalions of factory workers and shopkeepers carried the burden, and how two disgruntled soldiers betrayed the attack date weeks in advance. It follows the revisionist case made by modern historians, that the Somme destroyed the professional core of the German army and forced the retreat to the Hindenburg Line, and ends with the ghost soldiers who walked British streets a century later.

• How Falkenhayn's plan to bleed France white at Verdun turned the Somme into a British battle
• The Pals battalions: whole towns enlisting together, and what that meant on the casualty lists
• A surprise party with booby traps: the intelligence failures that gave away the date and place
• Why the Allies attacked anyway: Brusilov, Verdun, and the coalition timetable that severed the brakes
• Tanks, creeping barrages, and Operation Alberich: the case that the Somme broke the German army