The Battle of the Frontiers was a series of battles fought along the eastern frontier of France and in southern Belgium shortly after the outbreak of the First World War. The battles represented a collision between the military strategies of the French Plan XVII and the German Schlieffen Plan. The British Expeditionary Force (BEF) fought a relatively minor battle at Mons which, by virtue of its position facing the critical right wing of the invading German army, had a significance that far exceeded the number of men engaged or casualties inflicted. The defeat of the French offensive in the Battle of the Ardennes led to a general retreat to the Marne River where the French and British forces regrouped for the defense of Paris.