There was a secret that would win the war for France. Not a secret weapon, nor some surprise strategy that would send the enemies of France reeling in defeat, but a sleight of politics that would drive the British from Spain without a musket being fired. It was a secret that must be kept, and must be paid for.

To which end, on a pitiless winter’s day in 1813, two men climbed into the northern hills of Spain. Whenever the road forked they took the lesser path. They climbed by frost-hardened tracks, going ever higher into a place of rocks, eagles, wind, and cruelty, until at last, at a place where the far sea could be seen glittering beneath a February sun, they came to a hidden valley that smelt of blood.

There were sentries at the valley’s head; men wrapped in rags and pelts, men with muzzle-blackened muskets. They stopped the travellers, challenged them, then incongruously knelt to one of the horsemen, who, with a gloved hand, made a blessing over their heads. The two men rode on.

The smaller of the two travellers, the keeper of this secret of secrets, had a thin, sallow face that was pockmarked by the old scars of smallpox. He wore spectacles that chafed the skin behind his ears. He stopped his horse above a rock amphitheatre that had been made when this valley was mined for iron. He looked with his cold eyes at the scene below him. ‘I thought you didn’t fight the bulls in winter.’