In the early 20th century the German General Staff viewed the Schlieffen Plan, created by its Chief, Count Alfred von Schlieffen, in 1905, as a blueprint for avoiding a dreaded two front war by defeating France in six weeks or less. A speedy victory over France would allow the German Army to redeploy against Russia before the Tsar could fully mobilise his huge country. When Schlieffen retired in 1906 his successor, Helmuth von Moltke the Younger, modified the plan and executed it to near victory in the first month of the First World War. The German offensive was stopped just short of Paris at the Battle of the Marne, killing any hope of a quick victory. Count von Schlieffen's plan failed because of a combination of intrinsic flaws, poor modifications, and imperfect execution.
The plan focused on two central elements. Firstly, one section of the French border, between France and Belgium, was much more lightly defended than the rest (which was defended by the powerful Maginot line and the natural barrier of the Alps). Secondly, the German Army could mobilize more rapidly than the armies of the other European powers. The plan sought to exploit these two facts by calling for a sweeping attack by more than half of the German Army through Belgium, the Netherlands, and Luxembourg (violating the neutrality of these nations). This powerful German right wing would force a French surrender in 42 days by capturing most of the French army in an envelopment battle around Paris. The German Army would be free to complete its wide U-turn (it would end up facing back towards the German border) and would then be rapidly shifted to the Eastern front by rail.
However, when Count Alfred von Schlieffen devised his invasion plan it was merely a theoretical exercise. Though Kaiser Wilhelm II had asked for a plan that would allow the German Empire to fight an eventual two front war, Schlieffen's plan was less an actual battle plan and more a document calling for the addition of twenty divisions to the German Army. These necessary divisions were never added and the plan, though it was modified, was never significantly questioned, becoming the one and only invasion plan of the German General Staff.
This lack of alternatives constituted a serious problem because the Schlieffen Plan, partly because it was originally a theoretical document, left no room for error. Every German soldier was expected to march the maximum projected distance every day while brushing aside enemy opposition. Yet the average German soldier had, a few weeks earlier, been a civilian called up for service. It had perhaps been years since they had been trained and put in reserve, and there was no time for additional training because, as explained above, Germany's rapid mobilisation was one of its big wartime avantages. In addition the supply system, which relied heavily on horse drawn wagons since there weren't enough trains, was not nearly as efficient as predicted and therefore slowed the rate of advance even more. Instead of the 25 miles a day that the Schlieffen Plan necessited, the rate of march was almost half of that.
The plan also relied on a series of underestimations of the enemy's capabilities. For example, the plan allocated little time to the complete occupation of Belgium. Indeed the entire invasion of Belgium was viewed as basically a march, and the expected rate of advance was almost the same as that in friendly territory. In reality the Belgians proved to be a much tougher foe than the plan assumed, holding out tenaciously by defending cities and fortresses. In addition, the Germans didn't think that Britain would honour its alliance with Belgium, established by the London Treaty of 1839, and would instead remain neutral. The plan itself assumed that it would take the British two months to bring a significant force across the channel. Instead, the elements of the German Army advancing down the coast ran into the British Expeditionary Force (placed near the coast by the French General Staff because it assumed there would be no fighting in that section of the front). The 75,000 strong BEF further delayed the advance and forced them to turn away from the coast sooner than planned.
Moltke's modifications also contributed greatly to reducing the chances of German success. As well as placing an additional 180,000 soldiers in East Prussia, he decided to weaken the right wing (sweeping through Belgium) in order to strengthen the left wing (holding Alsace-Lorraine, a German territory since 1871). This lessened the power of the integral part of the plan. Von Schlieffen had predicted that the French would surge forward into Alsace-Lorraine upon the outbreak of war (which is exactly what they did), allowing the immensely powerful German right wing to envelope and destroy the French Army. He was willing to sacrifice territory on the left since this would draw the French armies away from Paris and since any lost territory would have been easily recovered by the envelopment. Moltke, however, was not willing to sacrifice territory and instead took men from the point of decision. In practice this meant that the French armies were kept closer to Paris and that the decisive attack was robbed of its strength.
The last nail in the coffin of the Schlieffen Plan was Moltke's decision, just before the Battle of the Marne, to move 80,000 soldiers to the Eastern Front. This decision was prompted by the surprising speed with which the Russians had started advancing into Germany (the Schlieffen Plan having once again underestimated the enemy). In any case the shift took even more soldiers from the point of decision. Two days before these reinforcements arrived the German forces in East Prussia delivered a resounding defeat to the Russians at the Battle of Tannenberg, virtually destroying the Russian Second Army and making the reinforcements completely pointless.
Overall, Moltke's reluctance to sacrifice territory both in Alsace-Lorrain and in East Prussia doomed the Count von Schlieffen's plan to failure. The German Army simply did not have enough men to hold the entirety of the German Empire's borders and to simultaneously mount a decisive attack to knock France out of the war quickly. Blame can of course also be attributed to the faulty assumptions of the Schlieffen Plan as well as to the Kaiser's focus on building up the German Navy, which robbed the Army in favour of a part of the military which would never be strong enough to compete with its rival, Great Britain. However, the execution of the Schlieffen Plan fell to Moltke and, in the end, he was the one who failed to secure a victory that lay within his grasp. Two months into the war the lines had stabilised into the trenches that would come to characterise the First World War.