The Longest Day was made in the fifties or early sixties. It was a very politically charged time with the Red Scare. To expect it to be completely accurate in all accounts is kind of silly. They did touch on the french resistance a good bit, though, so I don't think you have that much to complain about there.
What really made D-Day possible wasn't European intelligence, even though that helped, but it was the cracking of the German's secret code. The US sent false messages to the German's giving them the idea that they would be invading from England. They even went so far as to have the British inflate fake military vehicles along the English border too fool Rommel.
But I digress...
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