With dense forests and large ravines, the French and British thought this region was impassible?
The German plan to attack through the Ardennes and Belgium was called the Schlieffen Plan, and was first used in World War I. When Hitler's Generals unveiled their plan to attack France and the Low Countries in 1940, they simply dusted off this old Schlieffen Plan and updated it. In this way, Hitler's Blitzkrieg armies could bypass the heavily defended French Maginot Line and outflank their defenses.
Later in the war, in December of 1944, the Germans launched the Ardennes Counteroffensive (The Battle of the Bulge) using the same route through the forest once again, and once again, catching the Allies unprepared.
You do not tell us what war you are talking about, so it is hard to know for sure, but I think that you are probably talking about the Ardennes Forest.
Because the British and French thought that this forest was impassable, they thought there was no way that Germany could attack France through it. Therefore, they set up defense plans that did not cover that area. When the Germans attacked through the Ardennes in WWII, the Allies were caught unprepared.
This attack was part of the Blitzkrieg that Germany conducted so successfully at the start of the war.