The Franco-Russian Alliance made it clear that Germany would have to face both French and Russian forces if they were ever to get in a war with either one. Consequently, the Germans believed that they would destroy France before Russia had time to mobilised. Germany estimated that it would take both themselves and France two weeks to mobilized. On the other hand, the Germans believed that it would take Russia six weeks to muster an offensive counter attack against them. The reason the Germans believed this is because of the closer proximity of France and the technological inferiority of Russia.
As early as 1899, Schlieffen had developed a plan of attack against France. Schlieffen realized that Germany could not attack straight across their border due to the fortresses, which France had built along Alsace and Lorraine after 1870. Therefore, at the heart of Schlieffen plan was the idea that Germany would have to attack France by first going through Belgium. Because Belgium had been neutral since 1839, it was assumed to be an easy target, which would provide the Germans with quick access into France. Additionally, Schlieffen predicted that France would eventually attack through Belgium, so Germany might as well do so first.
Although Schlieffen died in 1912, Germany still used his plan when the war broke in 1914. As executed with certain modifications in the fall of 1914, the plan at first seemed likely to succeed. The swift German incursion into Belgium at the beginning of August routed the Belgian army, which abandoned the strongholds of Liège and Namur and took safety in the fortress of Antwerp. The Germans, rushing onward, then defeated the French at Charleroi and the British Expeditionary Force of 90,000 men at Mons, causing the entire Allied line in Belgium to retreat. At the same time the Germans drove the French out of Lorraine, which they had briefly invaded, and back from the borders of Luxembourg. The British and French hastily fell back to the Marne River, but three German armies advanced steadily to the Marne, which they then crossed. The fall of the French capital seemed so imminent that the French government moved to Bordeaux. After the Germans had crossed the Marne, however, the French under General Joseph Jacques Césaire Joffre wheeled around Paris and attacked the First German army, commanded by General Alexander von Kluck, on the right of the three German armies moving on Paris.
In the First Battle of the Marne, which took place on September 6-9, the French halted the advance of Kluck's army, which had outdistanced the other two German armies and could not obtain their support. In addition, the German forces had been weakened on August 25 when, believing the victory had already been won in the west, the German chief of staff, General Helmuth von Moltke, dispatched six corps to the eastern front. The French pressure on the German right flank caused the retreat of Kluck's army and then a general retreat of all the German forces to the Aisne River. These errors (which were sanctioned by General Headquarters) cost the Germans any further progress and they withdrew back to safe positions north of the Marne River, where they resisted attempts by the French to dislodge them.
The German commander, General Erich Von Falkenhayn, decided that his troops must hold onto those parts of France and Belgium that Germany still occupied. Falkenhayn ordered his men to dig trenches that would provide them with protection from the advancing French and British troops.
The Allies soon realised that they could not break through this line and they also began to dig trenches. After a few months these trenches had spread from the North Sea to the Swiss Frontier. For the next three years neither side advanced more than a few miles along this line that became known as the Western Front resulting in stalemate.
The fault lay not only with Kluck, but also with the German Commander-in-Chief, Count Helmut von Molkte and probably with the Schlieffen plan itself, which failed to account for the limitations of infantry formations operating at such rapid tempos.