After the Phoney War, the Battle of France began in earnest on 10 May 1940. To the east, the German Army Group B invaded the Netherlands and advanced westward. In response, the Supreme Allied Commander—French General Maurice Gamelin—initiated "Plan D" and entered Belgium to engage the Germans in the Netherlands. The plan relied heavily on the Maginot Line fortifications along the German–French border, but German forces had already crossed through most of the Netherlands before the French forces arrived. Gamelin instead committed the forces under his command, three mechanised armies, the French First and Seventh Armies and the British Expeditionary Force (BEF), to the River Dyle. On 14 May, German Army Group A burst through the Ardennes and advanced rapidly to the west toward Sedan, then turned northward to the English Channel, using "Generalfeldmarschall" Erich von Manstein's plan "Sichelschnitt" under the German strategy Fall Gelb, effectively flanking the Allied forces.