Final answer:
In the Battle of the Marne during World War I, the Ardennes' significance was demonstrated as a strategic blind spot, which led to the successful halt of the German advance by allied forces in 1914.
Step-by-step explanation:
The Ardennes region played a critical role during the First World War, particularly in the Battle of the Marne in September 1914. This crucial battle was a turning point where the French and British troops successfully halted the German advance towards Paris. The battle led to both sides establishing trench warfare lines which resulted in a protracted war of attrition. During this engagement, the strategic significance of the Ardennes was illustrated by the fact that the narrow, heavily wooded landscape was considered difficult to traverse and therefore not the main focus of defensive preparations, which the Germans used to their advantage in World War II. With the Germans quickly advancing through Belgium in World War I, the Ardennes was overlooked making it a strategic blind spot which they later aimed to exploit in World War II.