The Greatest Deception of All Time: Britain, Ultra, and D-Day
Luckily for the Allies, a significant amount of Ultra was being deciphered by the Allies in 1943 and 1944. This made it easy to measure the plan's effectiveness as it was being carried out. During the spring of 1944, Ultra was thorough enough to offer a nearly complete picture of the enemy. On May 29 (one week before D-Day) a message, regarding fuel allocations to Atlantic Wall construction projects, was intercepted by Bletchey Park. It started out by saying that the fuel allocations from OKW was:
approximately 20 percent less than applied for on basis of summer programme. There would be sufficient for the concrete envisaged in the programme that it could be carried out without considerable disturbance {emphasis mine}..... C-in-C West (von Rundstedt) therefore requests that as a precautionary measure a fuel reserve be made available for the construction.... recourse would only be had to this reserve if after the first ten days of June the situation can be reviewed as a whole.
[9]
Instead of being worried about the impending invasion, the Germans, as shown above, were as Ronald Lewin put it, "calmly discussing concrete and fuel and improvement of defenses as though unconscious of time and danger." [10] This complacency was deeply reassuring to the Allies.
On June 6, the Allies invaded Normandy with almost total surprise. Although some German troops put up heavy resistance, the German leadership acted as though nothing significant had happened. Hitler and his generals remained convinced that the landings were only a diversion, and that the real invasion (the Schwerpunkt) would take place later and in the Pas de Calais area. [11] The 15th Army (the largest army in Western Europe), instead of moving south to encounter the Allied forces, remained in Pas de Calais. Had this not been the case, the Allies might have faced a disastrous situation. [12] Thanks to Bodyguard, it did not happen.
Bodyguard was not the only reason, however, for D-Day's success. Among the others was the weather. Before the U-Boat threat was removed in May 1943, the Germans maintained several weather stations throughout the Atlantic. After May 1943, the Germans were largely ignorant of Atlantic weather patterns. In contrast, the Allies were aware of much of the weather on the Continent, mainly through Ultra encrypts. [13] This disparity was a distinct disadvantage for the Germans.
Because of Overlord's strict landing requirements, there were only three days on which the invasion could have occurred:
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