How does the inset map at the top of the page explain why hitler was expecting the invasion to cross from dover to calais over the strait of dover?

Hitler expected an invasion over the Strait of Dover because it is the geographical point with the shortest distance between Great Britain and Mainland Europe, more specifically: France.

Hitler did not expect an invasion through the English Channel to land on the Normandy coast, because the distance from Normandy to England is over 100 miles, the weather is cloudy and rainy for most of the year, and Normandy was heavily protected by the Nazi Army.

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