At the outset of World War I, both the French and the Germans sought to realize preconceived offensive strategies on the western front, respectively Plan XVII and Aufmarsch II. While the Germans immediately advanced into Belgium and Luxembourg the French, in line with their strategy, invaded Alsace and Lorraine. The Battle of Lorraine saw the decisive reversal of the French offensive, swiftly followed by a further defeat of the French at the Battle of the Ardennes. The Germans were now wholly on the offensive, achieving further victories at Charleroi and against the British Expeditionary Force at Mons. The Allied ‘great retreat’ was now in full flood, and the Germans were advancing on Paris. By early September the very speed and breadth of the German advance became their downfall. The French and British troops exploited the widening gap between the 1st and 2nd armies as they drew closer to Paris, halting the advance and forcing them to retreat to dugouts near the River Aisne.