The German chief of the general staff, Alfred Graf von Schlieffen, devised a plan for war on two fronts against France and Russia which could have ended the war within a few weeks. He correctly assumed that the French would attempt to regain Alsace-Lorraine, so he planned a major offensive against the French with 90% of the German army attacking through Belgium. The slower Russian forces would be controlled with his remaining troops.
However Schlieffen's successor, General Helmuth von Moltke was reluctant to violate Dutch neutrality so he decided to strengthen the forces defending Alsace-Lorraine leaving only 60% of German mobile field forces to attack France.
The French commander in chief, General Joseph J. C. Joffre, was relying on the British Expeditionary Force (BEF) to reinforce the French left flank. He was also depending on the ability of the Russian army to launch simultaneous offensives against Germany and Austria in the east, but Russian mobilisation could not begin for 3 months.