In October 1939 the Germans created ‘Fall Gelb’ (Plan Yellow), plotting an invasion of France via Belgium, avoiding the Maginot Line. In January 1940, a German plane carrying documents crashed in Belgium, forcing a change of tactics. Army Group B would invade through the Netherlands and Belgium, focusing the Allies’ attention away from action further south. The larger Army Group A would invade through the Ardennes, believed to be impenetrable by the Allies. France and Great Britain had assumed that Germany would recreate elements of the historic Schlieffen Plan, attacking through Belgium but then sweeping south to Paris. As a result, they placed their best forces deep in the Low Countries, leaving the Namur-Sedan gap poorly defended by weak French divisions. The German Blitzkrieg strategy took the Allies by surprise, creating chaos on the ground and from the air, allowing the Germans to make the critical advance across the Meuse and on into France.