Why did the Atlantic Wall fail to stop the D-day invasion?

Answer 1

There is no one reason why the Atlantic Wall failed to stop the D-Day invasion. It was due to many reasons, such as:

Field Marshall Erwin Rommel (the architect of the Atlantic Wall defenses) was never given enough resources to actually complete the main plan and had to resort to partial defenses that had fake batteries and personnel.

Rommel was relieved of duty before completion.

The allies attacked at Normandy rather than Calais as was believed to be the most likely invasion point (and hence the most fortified).

Special forces and insurgents created enough confusion that the defenders were unsure if it was the real invasion or a decoy. This is especially poignant because it was expected that Patton would lead the invasion but he was the true decoy.

Allied air and naval superiority.

The German generals were also restricted in the use of the Panzer Lehr, the German counterattack tank division, that was supposed to move to throw the allies back into the sea. Release of the Panzers was by direct command of Hitler himself and the order came too late.

Even with all these things going in the allies favor, it was still a tenuous thing at best until the breakout of the hedgerows (that were also part of the German defensive plan).