I believe that you are thinking about the Ardennes Forest, which is a region of forest and rough topography that is mainly in Belgium and Luxembourg (but is also partly in Germany and France). Germany attacked through the Ardennes in WWII because that was not what the French were expecting.
Because the terrain of the Ardennes and its forests were so difficult to travel through, the main Allied commanders before WWII did not believe that Germany would attack there. They defended to the north and the south of the Ardennes, but left that area practically undefended. They felt that modern armies with their needs for motor vehicles and massive supply trains could not come through that area. Of course, they were wrong. The German army was able to move through the Ardennes, break through the minor defenses they found, and spill out into France before the French could contain them. This was a major factor in the quick German victory over France.
So, I think that what you are asking about is the Ardennes, which is not really a pass into Western Europe but rather a region that did not seem like a good way to get from Germany to France and was therefore left undefended.