German military commanders made a series of fundamental mistakes which allowed Operation Overlord to succeed, according to a military historian.
Professor Jeremy Black said the German High Command made several "fundamental, pretty stupid mistakes" in preparing for the inevitable invasion and the Battle of Normandy.
He said the Nazis believed the Second Front would target Calais - taking advantage of the shortest crossing of the English Channel.
The Germans also did not have the equivalent of the British Ultra codebreakers at Bletchley Park and were "out-thought" by the Allies.
Prof Black, who teaches at the University of Exeter, said they were also too slow in counterattacking with Panzer tanks once the beach landings began.
"It was an appalling intelligence failure on the Germans' part," he said.
"To not realise where the largest armada in history, which was prepared within 50 miles of German-defended territory, was going to focus on is really appalling.
"It reflected the really bad position of German signals intelligence, they had no equivalent to Ultra, it reflected their lack of high-up implanted agents, it reflected very weak photo reconnaissance, which is in part on the fact the Allies had won the air war."
The Allies, through a decoy campaign named Operation Fortitude, had tricked the enemy into believing the main attack would come in the Pas de Calais, not Normandy.
The subterfuge gave the Allies their element of surprise when the invasion took place, tipping the balance in the Allies' favour and eventually leading to Germany's defeat.
"The reasons the Germans were really stupid was that the same thing had happened in 1943," Prof Black said.
"The invasion was launched in Sicily and the British had played up the idea of attacking Sardinia instead.
"The Germans had a track record in getting it wrong and you would have thought they would be a bit more thoughtful about it."
He went on: "One of the great mistakes they always make in these war films is to make the Germans appear really brilliant. By 1944 they certainly weren't.
"We've always been inclined to overrate the strategic skill of the Germans. They were pretty good tactically, they were not too bad operationally but they were always hopeless strategically.
"Repeatedly they got it wrong. The classic example is trying to take the Soviet Union out of the war, which in the end did for them. But also declaring war on the US was pretty stupid.
"There is a whole host of fundamental, pretty stupid mistakes which the Germans made."
