On June 6th, 1944 the Allies began the re-conquest of Europe from Hitler's Third Reich by launching an amphibious landing on the coast of Normandy in France. This was the biggest invasion force ever assembled in the history of war, but it was not sufficiently large enough to guarantee victory. The Germans had set up strong defensive positions all over Europe in anticipation of an assault and had many men standing in reserves to assist wherever the Allies landed.
The Allies realized that they needed to prevent the Germans from sending reinforcements from one position to the other. To prevent that from happening, the Allies needed to convince the Germans that the Normandy invasion was only one in a series of attacks that the Germans would have to defend.
That is why the Allies, especially the British, went to great lengths to provide the Germans with misinformation about the planned invasion. Using captured German codes and double agents, they sent messages to German intelligence indicating that the invasion army was much larger than it actually was and that the main attack would come at Calais in Northern France. They also played up the possibility of other attacks from Norway, Italy, the South of France, Greece, and the Balkans.
Because the Germans were not sure where the invasion would come, they had to keep all of their garrisons manned and could not concentrate their resources on any one spot. To what extent they did concentrate their forces, they concentrate in Calais, instead of Normandy where the attack was actually planned.
The ruse went far beyond simple misinformation, however. The Allies created two bogus armies including the First United States Army Group and they went to enormous lengths to make the Germans believe that they were actually full of thousands of men. Indeed, those armies were initially filled with men assigned to "real" armies preparing for Operation Overlord (D-Day).
In addition, British intelligence created inflatable tanks and plywood battleships to accompany the tent city and base where the army was supposedly housed. Care was taken to move the vehicles at night and add tire tracks when possible. Since the ships were in range of German guns on the coast, the few "real" people assigned to the FUSAG and its counterpart used fireworks to simulate damage.
While a close look at these fake armies would have made it clear that they were fake, they looked real enough for German surveillance planes flying high above Britain. The legitimate looking reconnaissance photos those planes took and the disinformation that the Germans were being fed by Allied intelligence convinced the Germans, including Hitler, that there were really three armies in Britain all preparing for an invasion.
The Germans might have suspected that it was all a ruse if the Allies had entrusted placed know-name generals at the head of the fake armies. That is why General George Patton, a man renowned for his toughness at the head of the First United States Army Group. The Germans regarded him highly and were convinced that he would lead the assault on Europe. Since his forces appeared ready to attack Calais, the Germans dared not move transfer any soldier from Calais to Normandy, where the attack actually happened.
Patton played his part well, although almost certainly despising it. He had been relieved of command after slapping and verbally abusing as cowards soldiers coming from the front lines with combat fatigue. His actions had caused such a controversy that, talented though he was as a military commander, he was removed form command. His reputation remained, however, and was instrumental in making the disinformation campaign successful.
All of the efforts at tricking the Germans paid off after the attack on Normandy was under way. Convinced by reports that said that the attack on Normandy was just a ruse to get the Germans to move men away from Calais in advance of the real Allied assault, Hitler refused to reinforce Normandy even after the attack on it began.
Weeks into the Allied assault and even after Patton had taken command of the Third Army, the Germans still kept men stationed at Calais. By that point, however, it was too late for those men to be of much help.
