Before his retirement in 1905, German Field Marshal Count Alfred von Schlieffen had concluded that, with Russia and France allied, Germany needed a plan to fight a two front war against those nations, and Germany had little chance of winning such a conflict if it became protracted. His plan for war therefore required two things: speed, and a Russian lack thereof.
The Schlieffen plan first assumed that in the event of war, the Russian army would be slowest among the nations to mobilize its army. This assumption was more than fair: Russia had the biggest, yet probably the least industrialized army among the European nations. The comparative lack of railway lines, locomotives and carriages, for example, proved an especially disadvantageous trait in moving their forces. Russia would probably need at least 6 weeks to put any amount of its military into Germany. In the meantime, Germany would be defeating France.
The Schlieffen plan called for at least 7 German armies to be placed on its western borders opposite (from North to South) Holland, Belgium, Luxemburg and France. These armies would move like a giant, synchronized wheel, moving counterclockwise through Holland, Belgium, Luxemburg and Northern France. Naturally, this required the Northernmost armies to travel the farthest, with each successively southern army travelling less. The final two armies, those stationed at the Franco-German border, would simply attack straight on, but only enough to ensure that the French could not transfer any of its troops to the North.
Schlieffen emphasized that the northernmost armies had to be kept the strongest, but if it could remain so, then any other weakness in the plan could be overcome. Within 6 weeks, Paris would be captured, the French army would be surrounded and would have to surrender, and the entire German army would be free to move to the Eastern front to face the Russians.
By 1914, the German chief of staff Helmuth von Moltke had altered the plan significantly, condensing the right wing to be opposite only Belgium. He reasoned that an invasion of Holland was both unwise and unnecessary, but the new plan perhaps took a greater risk than the original: the massive forces of the right wing would now all have to funnel through a 12-mile passage in the Ardennes (near the border of Belgium and Germany), and a quick strike through Belgium, including the reduction of its fortress at Liège, became absolutely essential to the success of the plan.
Moltke’s uncle had been a famously successful field marshall, and his plan for a German war against France and Russia was to simply allow the French to destroy themselves against German defenses in the rough terrain on the Franco-German border, while the rest of the German army slowly chewed up the Russians in the East. For this elder Moltke, the most the Germans could hope for in such a European war was a favorable negotiated settlement. Schlieffen obsessed with a plan to do better, his dying words being: “It must come to a fight. Only keep the right wing strong!” The younger Moltke was determined to do so.
On August 2, the first stages of the plan went into action: Cavalry stormed into Luxemburg to seize control of its rail networks. Germany demanded that Belgium give it unobstructed passage of its armies, which King Albert refused, instead ordering the destruction of all bridges and rail lines between Germany and Belgium. German eyes fixed upon the Belgian fortress town of Liège, which controlled the narrow passages through which the German right wing, the key to victory, had to pass.