Hoenmanns was standing in the snow, worrying what would happen if he was interred and the Luftwaffe mistakenly sent his mistress’s belongings to his wife, when his passenger, Major Helmuth Reinberger, announced that he was carrying one of the greatest secrets of the war—the plan for the long-rumored German offensive in the West.

By the time the plan arrived at the Belgian High Command that evening, it had survived two attempts by Reinberger to burn it, but, even scorched, it still had an important story to tell. The German offensive in the West would begin with a variation of the Schlieffen Plan, which had shaped the encounter battles of the Great War—a thrust through Belgium and into northern France. Four days later, the phone rang in the flat of Alexander Cadogan, Halifax’s number two at the Foreign Office. Cadogan looked at the clock: it was 3:45 a.m. When he picked up the receiver, the voice on the other end said, “Telegram from Brussels. Belgians expect invasion of Belgium, Holland, and Luxembourg today.” Cadogan was unable to fall back to sleep.

In its brief four and a half months of existence, the plan the Belgians retrieved from Hoenmanns’s plane had unnerved almost everyone who had come into contact with it, starting with the German High Command, who were first introduced to the plan on September 27, the day Warsaw fell. The setting was the new German Chancellery, whose austere lines and monumental size spoke of the Roman and Greek influences on its architect, Albert Speer; and its 480-foot-long Grand Hall, 17-foot-high bronze doors, and statues of nude Aryan athletes spoke of the Wagnerian influence on its principal occupant, Adolf Hitler. The old Chancellery next door had been preserved, but only for its metaphorical value. Entombed in decades of Berlin grime, it stood as a symbol of defeat and humiliation, of Versailles and French occupation, of breadlines and runaway inflation, while its glittering successor proclaimed “ein volk, ein reich, ein Führer.”

The first car to arrive at the Chancellery that morning held Hermann Göring. The reichsmarschall, who was colossal in everything—girth, ego, ambition, even bad taste—was wearing a gold-trimmed white uniform and a cap whose chinstraps disappeared into the folds of his neck fat. The other lead cars held General Walther von Brauchitsch, commander in chief of the German Army, and his naval opposite number, Grosadmiral Erich Raeder; von Brauchitsch was dressed in standard army field gray, Raeder in navy blue. The cars in the rear held a bevy of aides and advisers, their shoulders adorned with gold braids, their chests spangled with medals. For a moment the Chancellery courtyard echoed with the sound of car doors slamming shut; then the assemblage gathered itself up and marched down the marble corridors to Hitler’s office, which Life magazine had recently called the largest office in the world. Inside, the Führer awaited his guests in a simple gray uniform, under a mural illustrating the capital virtues—Wisdom, Prudence, Fortitude, and Justice.

During the meeting, Hitler introduced his guests to what was still the germ of an idea with a lecture on “general time.” Every day of quiet in the West gives Britain and France another day to mobilize their industries, to build up their armies, to cut further into Germany’s superiority in the air and on the ground. Therefore, said Hitler, Germany must strike in the West this year—1939. General George Thomas, director of the War Economy Department, was appalled. German steel production was currently running tens of thousands of tons below target each month, and there were serious shortages of gunpowder, vehicles, ammunition, and spare parts. Many of Hitler’s other guests agreed with Thomas; the plan was recklessly audacious. The armored and motorized divisions returning from Poland would take months to refit; airpower could not be brought fully to bear in autumn, due to the uncertainty of the weather; and the army was not ready to face a professional, well-equipped Western army. In Poland, some machine-gun units had refused to fire for fear of giving away their positions, and some platoons and companies had refused to attack unless goaded by an officer. These deficiencies would have to be corrected before the Wehrmacht could undertake a major campaign. In the collective opinion of the Supreme Command, Germany would be incapable of launching a decisive offensive in the West until the spring of 1942. Strike now, warned one general, and the cost will be four hundred thousand dead.

At a second meeting, on October 10, Hitler reframed his offensive plan for the generals. It was no longer just a clever strategic move designed to catch the Allies off guard. The offensive was now fundamental to Germany’s survival in a struggle with a Britain and France bent on its obliteration. Evoking “general time” again, Hitler warned that unless Germany struck soon, the Allies would press neutral Belgium and Holland into the war and, from bases in the Low Countries, Britain and France could mount an annihilating air campaign on the industrial Ruhr, the engine of the German war machine. Over the next six weeks, what had begun as a relatively modest offensive plan—an enveloping movement through Belgium and southern Holland—grew steadily more ambitious. Responding to the Luftwaffe’s demand for airfields on the Channel coast, the plan was expanded to include the occupation of all of Holland. The role of German forces in the Ardennes region was expanded as well. Instead of supporting the main thrust through the Low Countries with a limited penetration into eastern France, the Ardennes force would sweep westward across France and capture the Channel coast.