AT 10.30pm on the night of June 5, 1944, the BBC French service in London broadcast 187 code messages to French Resistance units across the whole of France. They were all different: “Les carottes sont cuites”; “Il’y a de l’eau dans le gaz”; “Méfie toi du toreador”.
But they all meant the same thing: “The hour of your liberation is at hand. D-Day is at dawn tomorrow. Rise and fight.”
For René Piron in the town of Romans, just below the western wall of the Vercors plateau, the night of D-Day was not a quiet one. “It was midnight. A violent banging on my door made me wake up with a start.
“I was scared stiff. But I had no choice. I opened the door – and let out a sigh of relief!” he recalled.
“It was our Lieutenant Jeannot, dressed in hiking clothes and standing ironically to attention... He told me, still half asleep, that the Allies would be landing at dawn this morning on the beaches of Normandy. He had to repeat the message to me twice before I got it. It took my breath away...
“At 5am, my sack on my back, I said goodbye to my wife and left home. She was very brave and uttered no syllable of complaint... I can see their faces now, my comrades, on that morning of June 6.
“Standing together in the darkness on the terrace in front of my house, waiting for the truck... I can see them still as they took their weapons out from the hiding places, assembled them and cleaned them for action, for all the world as if we were experienced soldiers ... I can see their faces – serious and determined.
“No bravado. Just the cold resolution a man feels when he must leave behind all that he loves, all that he lives for. His wife, his children, his possessions.”
And so it was all over France. As Allied landing craft were using the final hours of darkness to manoeuvre into position for their dawn run in to the Normandy beaches, French men (and some women) were slipping through the darkness down silent streets to knock on neighbours’ doors and tell them the hour had come; worried wives were saying goodbye to husbands they might never see again; and young men (and some not so young) were pulling on boots and hitching up rucksacks before quietly closing their front doors behind them and vanishing into the darkness.
Three days previously, on June 2, the heads of the British Special Operations Executive (SOE) and the American Office of Strategic Services (OSS), the two allied secret organisations charged with fomenting resistance in German occupied Europe, went to see General Eisenhower at his headquarters in Portsmouth, where the Allied fleets for D-Day were gathering.
The question they asked the Allied Supreme Commander was a simple one: What did he want the French Resistance to do when the Allies landed on the Normandy beaches?
The original plan had been to bring the Resistance in France to action in two waves. The first wave, consisting of the Resistance networks closest to the Normandy beaches, would rise on D-Day.
The remainder, further south, would stay hidden until the second southern landing which the Allies planned on the Mediterranean coast. But Eisenhower knew that success in Normandy would be touch and go.
He needed to keep the Germans guessing so that they would keep some forces in the south and not throw all at the Normandy beaches.
So he ordered “Tout le monde à la bataille” – everyone to the battle.
He ordered all the French Resistance networks to action across the whole of France, north and south.
He knew he would be risking the lives of thousands of young French fighters who would be wiped out by the Germans before he could get to them.
But he also knew that if he was to succeed in Normandy this was a gamble he had to take.
My book is the story of one of those units from its birth through its terrible suffering in defeat, through its days of privation and hunger hiding in the forests of the great French Alpine plateau of the Vercors near Grenoble, to its final victory against France’s German oppressors.
With 4,000 lightly armed maquisard fighters against 1,200 fully armed Germans, this was the largest resistance battle of the Second World War and featured the most brutal German reprisals in Western Europe.
This is a story of heroism and betrayal, and the foolish Generals and scheming political leaders, and the greatest British secret agent of the second world war, and a beautiful Polish countess who helped him (and became his lover).
She went on to become, some say, one of Ian Fleming’s lovers after the war and the model for Vespa Lind in his first book Casino Royale.
But above all, this is the hidden story of D-Day. We know almost everything there is to know about what happened on the beaches.
But almost nothing about the brave young French men and women who gave their lives for our success in Normandy, quite as much as those who fought and died on the beaches themselves.
• Paddy Ashdown is a former leader of the Liberal Democrats and author of The Cruel Victory: The French Resistance and the Battle For The Vercors 1944 published by HarperCollins, price £25.