Operation Barclay was the deception plan to support the invasion of Sicily, and was intended to convince the Germans that the Allies might be about to attack Corsica, Sardinia or Greece rather than Sicily.
The big problem for the Allies was that Sicily was the obvious next step after the end of the campaign in Tunisia. The Germans and Italians had air bases on Sicily that threatened the Mediterranean shipping lanes, while the Allies had a suitable base on Malta for their single engined fighters.
There were alternatives available to the Allies. Sardinia was also close to the shipping lanes, and no further from the Allied bases in Tunisia. An invasion of Sardinia and then Corsica would give the Allies a base for a potential attack straight at Rome, or even further north around Livorno or Genoa, or into the south of France. To the east an invasion of Greece might convince the Turks to enter the war on the Allied side, and could allow the Allies to cooperate directly with the Soviets in the Balkans.
The overall deception plan was based around an Allied invasion of the Balkans through Greece, to be carried out by the fictional British Twelfth Army. Hopefully the Turks would join the war, and together the British and Turks would advance into Bulgaria and join up with the Soviets. To support this operation western Crete would be attacked. There would also be a series of diversionary attacks, including raids on the south of France carried out by British and French troops under Alexander and an invasion of Sardinia and Corsica carried out by US troops under General Patton.
Some real operations were carried out to support this story. A number of beach raids were conducted, and calls were put out for Greek translators and French fishermen familiar with the south of France. In an attempt to conceal the date of any possible operation, the completion date for work going on in North African ports was placed in late July, two weeks after D-Day for Operation Husky.
The French supported the deception operation with one of their best double agents, code named ‘Gilbert’, a former Abwehr agent who had volunteered for a ‘stay behind’ mission in North Africa in order to escape from German control. He became operational on 10 June 1943, and successfully convinced the Germans that the invasion fleet was being formed at Bizerte. As a result the port was bombed by the Germans, leaving the real fleet, ninety miles away at Sousse intact.
The most famous deception operation related to Husky was Operation Mincemeat, famous as the ‘man who never was’. This saw a dead vagrant in Naval uniform placed in the sea off the Spanish coast, carrying ‘confidential’ documents relating to upcoming plans. The body went ashore, was found by the Spanish, and copies of the documents passed onto the Germans. These documents helped convince the Germans that the Allies were probably going to attack the south of France as a diversion, and make a main effort in Greece.
Operation Barclay probably helped divert German attention away from Sicily and southern Italy. It was unable to help the Allies achieve tactical surprise on Sicily, partly because of the preliminary invasion of Pantelleria, and partly because of the sheer size of the eventual invasion fleet, but it did help convince the Germans to keep more troops in Greece than they needed, so helped weaken the German position in Italy.