The Schlieffen Plan, prepared by German Chief of Staff General Alfred von Schlieffen (1833 to 1913) in 1905, was a secret plan of attack by German armed forces against France, should the two countries go to war. The objective was to quickly defeat France in the west to avoid facing Russia in the east at the same time. The plan required German forces to quickly move through the neutral Low Countries, avoiding the main French fortifications on the Franco-German border. The plan was to attack the French lines from the rear, capture Paris, and gain a surrender in six weeks.

The Schlieffen Plan was weakened in practical terms in 1914 (when the First World War began) by the new chief of staff, Helmuth Graf von Moltke (1848 to 1916). With fewer troops available than planned, logistical problems, and the enemy putting up unexpectedly strong resistance, the Schlieffen Plan failed, and the German advance was first halted and then pushed back at the 'Miracle of the Marne' in September 1914, a situation which ultimately led to the establishment of the Western Front as a relatively static line of opposing defensive trenches.

In the early 20th century, states across Europe were mutually suspicious of each other's intentions. In the pre-WWI Alliance system, two blocs emerged: the Triple Entente of Britain, France, and Russia versus the Triple Alliance of Germany, Austria-Hungary, and Italy. In case war broke out between these two blocs, military strategists were tasked with formulating suitable plans of attack. Russian generals formulated plans on how best to attack Austria-Hungary, while French generals formed attack plans for a campaign against Germany. Most famous in history is Germany's plan of attack on France: the Schlieffen Plan.

General Alfred von Schleiffen, Chief of the German General Staff, dreamed up a plan on how best to face the French army on the Franco-German border. Completed by 1905, Schlieffen retired in December of that year, stressing that his plan was unlikely to be successful in reality since Germany lacked the necessary manpower. The plan involved certain other assumptions. The first was that Germany would probably find itself at war with both France and Russia at the same time. Britain would likely be the ally of France and Russia. This assumption was based on the treaty obligations of these states at the time and is what actually happened. The second assumption was the geography that had to be covered, and both the antiquated nature of the Russian army and its command structure meant that it would only be able to mobilise relatively slowly. Consequently, Germany's Eastern Front would require fewer troops to defend it in the initial stages of a pan-European war. It was estimated that Germany might have six weeks to deal with France before Russia's army was fully mobilised. Initially then, the bulk of the German Army could be deployed on the Western Front against France. If Germany could achieve a quick knockout blow in the west, then its whole army could be redirected to the Eastern Front.

The key to the Schlieffen plan was for German troops to move through neutral Netherlands, Luxembourg, and Belgium, thereby circumventing France's defensive fortifications on the Franco-German border. The German generals were convinced that their French counterparts, if given total freedom of movement, would mobilise the French army to attack and reclaim Alsace-Lorraine. The German generals would entice just such an attack by launching a feint advance in that region, but then swiftly withdrawing. This ploy would distract the French while the real German attack, consisting of 53 divisions, sped through the Low Countries and attacked French territory in a wide arc from Rouen to Paris, then crossed the Marne and Seine rivers to attack the French army from behind in the areas of Verdun, Nancy, Epinal, and Belfort. Part of this ambitious plan was to capture Paris, which would, it was hoped, bring about the fall of the French government and the capitulation of the state's armed forces. Once the French had been defeated, the German troops would be transported by railway to the Eastern Front to face the army of Russia.

By 1914, Moltke had made several alterations which "certainly weakened" the Schlieffen Plan (Bruce, 342). Moltke was unwilling to march through the Netherlands, and so the front of attack was narrowed, part of the invading force going through Luxembourg and the bulk through Belgium (between Antwerp and Liège). This decision slowed the advance and hampered logistics as seven German armies attacked the frontiers. Schlieffen had carefully calculated that a wider front was required in order to move sufficient manpower into France at the speed required. More significantly, Moltke reduced the right wing of the attack by one-third, equivalent to around 1.5 million men. Schlieffen had been most particular that for the plan to work at all, the right wing had to be immensely strong since it was performing the widest arc of the front. But Moltke feared early territorial losses to the French in the border region of Alsace-Lorraine and in East Prussia, should Russia mobilise quicker than expected in the east. Both of these eventualities did occur. The 1.5 million soldiers pared off from the Schlieffen plan were dispatched to these two alternative fronts. Troops were also siphoned off the main Schlieffen attack to deal with the more troublesome fortresses the invasion came up against, notably those at Antwerp. Moltke's caution, and the fact that Russia mobilised relatively quickly, reaching German territory in just 15 days, were to prove decisive in the Schlieffen Plan's failure.

It is significant perhaps that Moltke did not himself believe the Schlieffen Plan could really win a quick victory; he had once stated to the kaiser that a coming war in Europe "will not be settled by a decisive battle but by a long wearisome struggle with a country that will not be overcome until its whole national force is broken" (Strachan, 134). Moltke was quite right.

The German attack on the ground was preceded by a flurry of diplomatic activity. Permission was asked for German forces to move through Belgium on 2 August; Belgium refused. On the same day, German troops moved through Luxembourg. Britain had hoped to remain neutral in a continental war but was obliged to uphold Belgian neutrality via a treaty the two states had previously signed. In any case, Britain could not stand by and see France crushed and a new Europe formed that was totally dominated by Germany, its greatest economic and military rival in Europe. Britain informed the German government that a mobilisation through Belgium would result in Britain declaring war on Germany. Britain's unexpected involvement (from the German point of view) meant that now the war could not be a short one, since, even if France suffered severe military defeats, it would, with Britain as an ally, be very unlikely to simply surrender. On 3 August, German troops marched through Belgium anyway, and Germany formally declared war on France (and vice versa). On 4 August, Britain declared war on Germany. Meanwhile, Russia and Austria-Hungary had declared war on each other as they each wanted to control the Balkans. The First World War had begun, and other nations would soon join it, such as Japan and, eventually, the United States.

The Schlieffen Plan, although starting well and gaining Germany a sizeable slice of France, ultimately failed in its objective of achieving a quick victory on the Western Front. The German forces led by General Alexander Kluck (1846 to 1934) passed through Brussels easily enough but were held up by unexpected Belgian resistance at Liège's ring of 12 forts. German planners had estimated that with the attacking force armed with massive 'Big Bertha' 42-cm (16.8-inch) Krupp artillery guns, only two days would be required to destroy Liège's defences; it actually took ten days. Thanks to superior numbers and artillery, the German forces did defeat the British Expeditionary Force (BEF) at Mons and at Le Cateau, but these victories were costly in terms of time, material, and casualties. Kluck then improvised on the Schlieffen Plan, and instead of encircling Paris, he decided to pursue the retreating enemy. Chasing the French Fifth Army, Kluck exposed his right flank to the garrison of Paris, a move revealed to the Allies by air reconnaissance. Moving to face this new attack on his flank, Kluck then left an unusually large gap between the German First and Second Armies, a gap that was quickly exploited by both the BEF and the French armies led by General Joseph Joffre (1852 to 1931). The bold curving arrows of the original Schlieffen Plan had now turned into a mess of multidirectional attacks, counterattacks, outflanking maneuvers, and encirclements.

As the Schlieffen Plan fell apart, German command communications broke down badly. The German troops were now suffering from fatigue and poor supplies. French and British forces rallied in the Battle of the Marne, or what became known as the 'Miracle of the Marne', in September. The ever-cautious Moltke decided to withdraw, and the front was pushed back, finally establishing itself along the Aisne River. The Schlieffen Plan was in tatters, exposed as an overly optimistic estimation of the enemy's reactions and capabilities on the ground, as well as a gross underestimation of the logistical and communication demands such a large advance made on the German armies involved.

By the winter of 1914 and after a series of failed outflanking manoeuvres (known as 'the Race to the Sea') conducted by both sides, the Western Front stretched from Ypres near the Belgian coast to the Swiss border in the south. Both sides were obliged to build systems of trenches to better protect their troops from enemy fire. A long stalemate followed, with neither side ever making very much progress against the other over the next four years. This was exactly the situation the German generals had feared as Russia entered the war, and so, despite all the planning, they found themselves fighting on two massive fronts, a situation that ultimately cost them the war, even though Russia withdrew from WWI after the Bolshevik Revolution in 1917.