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The meme refers to the various intelligence campaigns by the Allied powers under the umbrella of “Operation Fortitude”. Operation Fortitude was a campaign of deception designed to spread German and Axis forces across the Atlantic so that the real invasions could be supported. Calais was one such sight, which made the Germans move troops there as opposed to Normandy, where the landing was actually set to take place.
Sources:
https://www.english-heritage.org.uk/visit/places/dover-castle/history-and-stories/d-day-deception/