This country fortified its borders with germany except for an area called the ardennes forest which they assumed could not be penetrated by a large army.

I think the answer is France. Because of the Ardennes Forest, Germany was not able to invade France from its east border. So Germany decided to attack Netherlands, Luxumbourg, Belgium, Norway, and Denmark. Hitler was able to go around the French border through the English Channel. He was able to defeat France with his strategy.