During the first years of the twentieth century, the great military powers of Europe established a system of mutual defense alliances that attempted to retain a balance of power in Europe. In the face of rising German economic and military power resulting from its unification and its victory in the Franco-Prussian War, France, Britain, and Russia formed the Triple Entente against the Triple Alliance of Germany, Austria-Hungary, and Italy. The assassination of Austria’s Archduke Franz Ferdinand by a Serbian nationalist secret society precipitated the issuance of ultimatums and the implementation of army general mobilizations. In the summer of 1914, the belligerent countries lacked a great statesman who could have steered history onto a different course.
The French entered the war believing in the superiority of taking the offensive (offensive à outrance) and its commanders subscribed to Plan XVII, which held that the next war would be fought along the common French-German border. According to the Plan, the two southern-most French armies would advance into German territory to reclaim Lorraine and occupy the Saarland, while three others would guard against German moves into Belgium east of the Meuse River. The British Expeditionary Force (BEF) would occupy the left flank near the border fortress of Maubeuge, leaving the 160 km gap to the Channel ports defended by four weak French territorial divisions.
German military planners feared fighting a two front war - France in the west and Russia in the east. Their mobilization plans relied heavily on execution of the 1905 Schlieffen Plan, which called for the rapid defeat of French and Belgian armies before slow-to-mobilize Russia could transport its huge army against East Prussia. Schlieffen’s plan called for victory in the west within 39 days before turning to face Russia in the east. German armies would be transported to the borders by a militarized railroad system and arranged with a preponderance of strength on the right (northern) flank. These northern ‘hammer’ forces would execute a sweeping hook through Belgium, encircling Paris from the west and trapping the five French armies and the BEF against the ‘anvil’ of German forces to the south. The plan was overly optimistic, lacking - as its creator pointed out - the superior numbers necessary to execute it and ignoring the responses of the enemy. Despite its shortcomings, it almost worked.
See: Schlieffen Plan