Since 1894 when France and Russia had concluded a military alliance, Imperial Germany had faced a nearly insoluble strategic problem. Further this problem was existential for the Second Reich. If the Fatherland’s military leadership could not determine a way to fight and win a two front war against both Russia and France and perhaps Britain, all nations nearly as powerful as Germany, the empire might very well cease to exist and would certainly be reduced second tier power status.
The plan arrived at by the vaunted Imperial German General Staff under the leadership, of; first, Count Alfred von Schlieffen and then after 1905, Helmuth von Moltke the Younger was called the Schlieffen Plan after its first and primary patron. Although the Schlieffen Plan was later significantly altered by Moltke it still maintained the name Schlieffen Plan in popular usage, although it should have been more correctly called the Moltke Plan.
The basics of the original Schlieffen Plan were that upwards of eighty-five percent of the German Army would crash through Holland, Belgium and Luxemburg, while the remaining fifteen percent of the German forces would hold Alsace-Lorraine and East Prussia. This massive “right hook” would then wheel south, destroying the French Army in a battle of annihilation, or Vernichtungschlacht, somewhere in the Somme, Oise or Marne River valleys. The Germans would go on to capture Paris, forcing terms on the beaten French. After the victory in the west, most forces would move quickly east to face Russia. According to the plan the defeat of France would take about six weeks. This would essentially allow Germany to, instead of fighting a two front war, to fight two wars in sequence; first, one against France and then a separate war against Russia.
When Moltke took over as Chief of Staff in 1905, he made several changes to the Plan. Most significantly he declared that Holland would not be invaded, leaving it as Germany’s “windpipe” to the world. This required that the German forces to be funneled thought the relatively narrow front of Belgium and Luxemburg and shifted the German main effort or Schwerpunkt, more generally south. On the eastern front, Moltke made no changes. East Prussia would still be held against the Russians by only the Eighth Army.
The Schlieffen Plan was predicated on four, what looked liked at the time to be reasonable assumptions. First assumption: the Russians would be slow to mobilize their full power. Second assumption: if the Czar’s armies did manage to mobilize quickly, they would be slow to move on to the attack. Third assumption: Austria-Hungry, Germany’s primary ally would focus on fighting the Russians; therefore the Russians would have to concentrate on Austria-Hungary to the east of the Carpathian Mountains, not on Germany in East Prussia. Last assumption: the war in the west would be over quick enough that the main German forces would be able to move east before the Russians could become a threat to the German heartland. As events worked out, all of these assumptions turned out to be false.