The Schlieffen Plan (German: Schlieffen-Plan, pronounced [ʃliːfən plaːn]) was a 1905 German General Staff thought-experiment which later became a deployment-plan and set of recommendations for German Commanders to implement as they saw fit using their own initiative. It was adopted as Deployment Plan Aufmarsch I in 1905 (later Aufmarsch I West) and modelled a Franco-German war which would not involve Russia but was expected to include Italy and Austria-Hungary as German allies. "[Schlieffen] did not think that the French would necessarily adopt a defensive strategy" in such a war, even though their troops would be outnumbered "but he recognized that this would be their best option and it therefore became the central theme of his analysis." Consequently, Aufmarsch I stated that Germany would have to go on the offensive to end this kind of war (on favourable terms) and in it, all of the German army was deployed on the German-Belgian border so it could launch an offensive into France through the southern Dutch province of Maastricht, Belgium, and Luxembourg. The deployment plan assumed that Italian and Austro-Hungarian troops would defend Alsace-Lorraine.
The French General Staff Plan XVII and the German Aufmarsch I were just deployment plans and not operational plans, though both were designed with specific operations/campaigns in mind. The operations that followed from the German General Staff's deployment plans, including Aufmarsch I, had no set durations or timetables because it was assumed that the timing of the operations would be decided by Army-level Commanders (each commanding c.100,000 combat troops) carrying out the overall plans under their own initiative. Schlieffen thought that the offensive operation following from Aufmarsch I could force the smaller French Army to commit itself to a Decisive Battle in which much of it might be destroyed for fear of the German army breaching their 'second defensive area' (the river Marne and the fortress-sectors of Verdun and Paris). If the French Army was defeated in battle it would be weakened, but if it denied battle then the defensive value of the 'second defensive area' could be greatly reduced. Building on this decisive first operation/campaign, further operations/campaigns would eventually bring about the defeat of France.
Helmuth von Moltke the Younger succeeded Schlieffen in 1906 and became certain that an isolated Franco-German war was impossible, due to shows of Franco-Russian solidarity during the Moroccan and Bosnian crises. Moltke also became convinced that Italy would not join in, due to the increasing Italian-Habsburg enmity and the anticipation of British entry into a Franco-German war, in which the Italian economy would be highly vulnerable to blockade. Under Moltke Aufmarsch I was retired, but in 1914 he attempted to apply the offensive strategy of Aufmarsch I West to the deployment plan Aufmarsch II West. This plan was designed for a two-front war and so reduced the forces available in the west by a fifth, meaning that the German offensive was too weak to succeed:
From his assessment of French defensive capability Schlieffen concluded that the German army would need at least 48.5 corps [1.36 million combat troops] to succeed with an attack on France by way of Belgium, but Moltke planned to attack through Belgium with just 34 corps [970,000 combat troops] at his disposal in the west. The Schlieffen plan [sic] amounts to a critique of German strategy in 1914 since it clearly predicted the failure of Moltke’s underpowered invasion of France. [...] Moltke followed the trajectory of the Schlieffen plan, but only up to the point where it was painfully obvious that he would have needed the army of the Schlieffen plan to proceed any further along these lines.
German Deployment Plans, 1905–1914
Aufmarsch I West
Aufmarsch I West anticipated an isolated Franco-German war in which Germany might be assisted by an Italian attack on the Franco-Italian border and Italian and Austro-Hungarian forces in Germany. It was assumed that France would be on the defensive because their troops would be (greatly) outnumbered. Therefore, to win the war on favourable terms Germany and her allies would have to attack France. After the deployment of the entire German army in the west they would launch an offensive through Belgium-Luxembourg with virtually all the German army. They would rely on an Austro-Hungarian and Italian force, based around a cadre of German troops, to hold the fortresses along the Franco-German border. Aufmarsch I West looked less feasible as the strength of the Franco-Russian alliance was made clear and Britain aligned with France, making Italy unwilling to support Germany. Aufmarsch I West was scrapped when became clear that an isolated Franco-German war was impossible and that Germany's allies would not intervene even if one did occur.
Aufmarsch II West
Aufmarsch II West anticipated war between the Franco-Russian Entente and Germany, with Austria-Hungary supporting Germany and Britain perhaps joining the Entente. Italy was only expected to join Germany if Britain remained neutral. 80% of the German army would operate in the west and 20% in the east. France and Russia would attack simultaneously, because they had the larger force and Germany would execute an "active defence", in at least the first operation/campaign of the war. German forces would mass against the French invasion force and defeat it in a counter-offensive, while conducting a conventional defence against the Russian force but rather than pursue the retreating French force over the border, 25% of the German force in the west (which amounted to 20% of the German army) would be transferred to the east, for a counter-offensive against the Russians. Aufmarsch II West became the main German deployment plan, as the Franco-Russian Entente expanded their armies and the German strategic situation deteriorated before 1914, Germany and Austria-Hungary being unable to increase their military spending to match them due to legislative deadlock.
Aufmarsch II West is implemented in August 1914 but using the overall strategy of Aufmarsch I.
Aufmarsch I Ost
Aufmarsch I Ost anticipated war between the Franco-Russian Entente and Germany, with Austria-Hungary supporting Germany and Britain perhaps joining the Entente. Italy was only expected to join Germany if Britain remained neutral. 60% of the German army would deploy in the west and 40% in east. France and Russia would attack simultaneously, because they had the larger force and Germany would execute an "active defence", in at least the first operation/campaign of the war. German forces would mass against the Russian invasion force and defeat it in a counter-offensive, while conducting a conventional defence against the French force but rather than pursue the retreating Russian force over the border, 50% of the German force in east (about 20% of the German army) would be transferred to the west, for a counter-offensive against the French attack force. Aufmarsch I Ost became a secondary deployment plan, as it was feared a French invasion force could be too well-established to be driven from German soil or at least inflict greater losses if not defeated sooner. The counter-offensive against France was also seen as the more important operation, since the French were less able to replace losses than Russia and it would result in a greater number of prisoners being taken.
Aufmarsch II Ost
Aufmarsch II Ost provided for an isolated Russo-German war, in which Austria-Hungary might support Germany. This plan provided for France being neutral at first and possibly attacking Germany later. If France helped Russia then Britain might join in and if it did, Italy was expected to remain neutral. About 60% of the German army would operate in the west and 40% in the east. Russia could begin an offensive because of its larger army and in anticipation of French involvement but if not, the German army would attack. After the Russian army had been defeated, the German army in the east would pursue the remnants. The German army in the west would stay on the defensive, perhaps conducting a counter-offensive but without reinforcement from the east. Aufmarsch II Ost became a secondary deployment plan, as the international situation made an isolated Franco-German war impossible. Aufmarsch II Ost had the same flaw as Aufmarsch I Ost, in that it was feared that a French offensive would be more difficult and/or costly to defeat, if not countered with greater force, either slower as in Aufmarsch I Ost or with greater force and quicker, as in Aufmarsch II West.
Marne Campaign, 1914
Holmes supported a view by Zuber, that Schlieffen had demonstrated in his thought-experiment and in Aufmarsch I West, that 48.5 corps [1.36 million combat troops] was the minimum force necessary to win a decisive battle against France or take strategically important territory and questioned why Moltke attempted to achieve either objective with 34 corps [970,000 combat troops], only 70% of the minimum required. In the 1914 Marne Campaign, the retreat by the French army denied the Germans a decisive battle, leaving the Germans to breach the secondary fortified area of Verdun–Marne–Paris. If this strong defensive position could not be overrun in the opening operation/campaign of the war, the French would be able to reinforce this line with field fortifications. This would force the Germans to breach the reinforced line in the opening stages of the next German offensive operation/campaign, an action that would be very costly. Holmes wrote that
Schlieffen anticipated that the French could block the German advance by forming a continuous front between Paris and Verdun. His argument in the 1905 memorandum was that the Germans could achieve a decisive result only if they were strong enough to outflank that position by marching around the western side of Paris while simultaneously pinning the enemy down all along the front. He gave precise figures for the strength required in that operation: 33½ corps [940,000 combat troops], including 25 active corps [active corps were units capable of attacking and reserve corps were units with only just enough weaponry and training to defend themselves]. Moltke’s army along the Paris–Verdun front consisted of 22 corps [620,000 combat troops], only 15 of which were active formations.
Lack of troops manifested itself as "an empty space where the Schlieffen Plan requires the right wing [of the German force] to be." In the final phase of the operation/campaign called for by the Schlieffen Plan, the German right wing was supposed to be "outflanking that position [the Verdun–Marne–Paris line] by advancing west of Paris across the lower Seine" but in 1914 "Moltke’s right wing was operating east of Paris against an enemy position connected to the capital city... he had no right wing at all in comparison with the Schlieffen plan." Breaching the Paris–Marne–Verdun line was impossible with the forces available to Moltke, something he should have appreciated.
Holmes could not adequately explain this monumental oversight but wrote that Moltke's preference for the tactical attack was well-known and thought that unlike Schlieffen, Moltke was an advocate of the strategic offensive,
Moltke subscribed to a then fashionable belief that the moral advantage of the offensive could make up for a lack of numbers" on the grounds that "the stronger form of combat lies in the offensive" because it meant "striving after positive goals.
the German offensive of 1914 failed, as the French evaded a decisive battle and retreated to the Verdun–Marne–Paris line. German territorial gains were reversed by a Franco-British counter-offensive, against the weak and outnumbered First and Second armies, on the German right/western flank during the Battle Of The Marne. Stahel wrote that the Clausewitzian culminating point of the German offensive (a theoretical point at which the strength of a defender surpasses that of an attacker) occurred before the battle, because the German right/western flank armies on the east side of Paris, were operating 100 kilometres (62 mi) from the nearest rail-head, requiring week-long round-trips by unfed and exhausted supply horses, which led to the right wing being disastrously short of ammunition. Stahel wrote that contemporary and subsequent German assessments of Moltke's implementation of Aufmarsch II West in 1914, saw no problem with the planning and supply of the offensive operation/campaign, even though these were instrumental to its failure:
In the eyes of many later German commanders (including Guderian who fought in the battle of the Marne), the German failure was not a reflection of an overly ambitious campaign objective or the prevailing battlefield conditions; it was the result of excessive caution and a failure to press the attack on Paris with every possible means in the hope of clinching the decisive success. The lesson seemed justified by the first campaigns of World War Two and formed a new cult of the offensive which subsequently pushed the German armies well beyond their limits in Operation Barbarossa.
The Schlieffen myth
Most of the pre-1914 planning of the German General Staff was secret and documents were destroyed but guesses have passed into public discourse. In the 1990s the discovery of RH61/v.96 (a 1930s study of pre-war German General Staff war planning, using incomplete records) and other documents made available by the fall of the German Democratic Republic, made an outline of German war-planning possible for the first time, proving many of the guesses wrong. A fundamental inference that all of Schlieffen's war-planning was offensive, came from the extrapolation of Schlieffen's writings and speeches on tactical matters to the realm of strategy,
There is no evidence here [in Schlieffen's thoughts on the 1901 Generalstabsreise Ost (eastern war game)] – or anywhere else, come to that – of a Schlieffen 'credo' dictating a strategic attack through Belgium in the case of a two-front war. That may seem a rather bold statement, as Schlieffen is positively renowned for his will to take the offensive. The idea of attacking the enemy’s flank and rear is a constant refrain in his military writings. But we should be aware that he very often speaks of an attack when he means a counter-attack. Discussing the proper German response to a French offensive between Metz and Strasbourg [as in the later 1913 French deployment-scheme Plan XVII and actual Battle of the Frontiers in 1914], he insists that the invading army must not be driven back to its border position, but annihilated on German territory, and 'that is possible only by means of an attack on the enemy’s flank and rear'. Whenever we come across that formula we have to take note of the context, which frequently reveals that Schlieffen is talking about a counter-attack in the framework of a defensive strategy [italics ours].
The most significant of these errors were that it was assumed to model a two-front war (against both France and Russia) and be Germany's only deployment/'war' plan, whereas both the thought-experiment and later deployment plan modelled an isolated Franco-German war (albeit with aid from Germany's allies) and the 1905 plan was one of three and later four plans available to the German General Staff. Lesser errors were that the plan modeled the total defeat of France in a single operation/campaign in under forty days and that Moltke The Younger foolishly weakened the attack-force by being overly-cautious and strengthening the defensive forces in Alsace-Lorraine. In truth Aufmarsch I West had the more modest aim of forcing the French to choose between losing territory or committing the French army to a 'Decisive Battle' in which it could be weakened and then finished off by later German campaigns
The plan was predicated on a situation when there would be no enemy in the east [...] there was no six-week deadline for completing the western offensive: the speed of the Russian advance was irrelevant to a plan devised for a war scenario excluding Russia.
and Moltke made no more alterations to Aufmarsch I West but came to prefer Aufmarsch II West instead - though he attempted to apply the offensive strategy of the former to the latter. 
Several historians using the pre-1990s understanding of the plan and its details, wrote that the plan was impractical, due to advances in weaponry, improved transport brought about by the industrial revolution and the rise of industrial warfare. Liddell Hart praised the Schlieffen Plan as a conception of Napoleonic boldness but that
The plan would again become possible in the next generation—when air power could paralyse the defending side’s attempt to switch its forces, while the development of mechanized forces greatly accelerated the speed of encircling moves, and extended their range. But Schlieffen’s plan had a very poor chance of success at the time it was conceived.
and Cohn wrote that the plan may have worked if Moltke had followed Schlieffen's original plan. Had Moltke not depleted the right flank on the Western Front, the 1st Army would not have been forced away from the sea, the British Expeditionary Force (BEF) would have been overwhelmed and the French armies would have been trapped between Paris and the French eastern frontier. That supply difficulties would have prevented this was contradicted by the German supply improvisations that occurred.
Fromkin, Stevenson and Zuber, wrote that what became known as the Schlieffen Plan, may not have been a plan but was laid down in a 1905 hypothetical memorandum and a brief 1906 addition. According to this school of thought, Schlieffen may not have intended his concept to be carried out in the form laid down but considered it an intellectual exercise. Fromkin wrote that that the memorandum had never been refined into an operational programme. No orders or operational details were appended and the memorandum acknowledged that for the plan to work, the German army needed more divisions and more parallel roads through Belgium. By ascribing much of the detail of the plan as it was implemented on Moltke, who had seen the memorandum and believed it to be an operational plan, which he expanded on, Fromkin referred to the "Moltke Plan" as opposed to the "Schlieffen Plan", as it may have been more a product of Moltke misreading the Schlieffen Memorandum of 1905 and its 1906 codicil.
Zuber wrote that the Schlieffen memorandum was a "rough draft" of a plan to attack France in a single front war, which could not be regarded as an operational plan, as the memo was never typed up, was stored with Schlieffen's family and envisioned the use of units not in existence. The "plan" was never published after the war, when it was being called an infallible recipe for victory which Moltke ruined. Zuber wrote that if Germany faced a war with France and Russia, Schlieffen's real plan was for defensive counter-attacks.
Palmer wrote that closer inspection of the extant records of the German war plan, reveal that the changes made by Moltke were small, that the plan was fundamentally flawed and that the Schlieffen plan did not deserve a high reputation, because it underestimated the speed and capacity of the Russian, French, British and Belgian armies.
Another view is that Palmer and Fromkin are correct. Palmer's opinion that the Schlieffen Plan was a poor plan was supported by it not being fully vetted. The Schlieffen Plan could have been a catalyst for operational thinking and planning and became the working name for a strategy of bypassing the bulk of the French forces through a flanking manoeuvre. Keegan, summarizing research on the Schlieffen Plan debate, criticized the plan for lack of realism about the speed with which the right wing of the German army could wheel through Belgium and the Netherlands, to arrive outside of Paris on schedule. Regardless of the path taken, there were not enough roads for the masses of troops planned to reach Paris in the time required. The Plan required German forces to arrive on time and in sufficient force; only one or the other could be achieved.
Keegan also wrote that the Schlieffen Plan was a leading example of separation between war planning and political-diplomatic considerations, which was one of the causes of the war. Schlieffen conceived the plan as the best possible solution to a strategic problem, while ignoring the political reality that violating Belgian neutrality, was the thing most likely to expand the conflict. The rigidity of the plan has also been a source of much criticism. The plan called for the defeat of France in 42 days, with an inflexible timetable, the German General Staff was unable to improvise in the "fog" of war; the Schlieffen Plan was anti-Clausewitzian in concept. During and after the war a deception was practised. Records were lost and material fabricated for a false alibi, to make more palatable the conduct of the men who made the decisions which led to a lost war.
- Manstein Plan (Second World War plan with some similarities)
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