Following Austria-Hungary’s declaration of war on Serbia, Europe’s intricate alliance system rapidly transformed a regional conflict into a continental one. Russia began mobilising its vast army on 30 July 1914 in support of Serbia. Germany, honouring its alliance with Austria-Hungary, viewed Russian mobilisation as an act of war and issued an ultimatum demanding that it cease. When the ultimatum expired, Germany declared war on Russia on 1 August. Two days later it declared war on Russia’s ally, France.
For German military planners, this was the nightmare they had long feared: a two-front war. France lay to the west, Russia to the east. Fighting both simultaneously would stretch Germany’s resources to breaking point. Although Russia possessed an enormous army, its mobilisation was expected to take several weeks because of its vast territory and comparatively underdeveloped railway network. Germany therefore believed it had a short window of opportunity to defeat France before turning its full attention eastward.
The strategy had been devised almost a decade earlier by Field Marshal Count Alfred von Schlieffen, Chief of the German General Staff until 1906. Schlieffen argued that France’s eastern frontier was protected by an extensive chain of modern fortresses stretching from Switzerland to Luxembourg. A direct assault would likely become bogged down in costly siege warfare. Instead, he proposed an enormous sweeping manoeuvre through the neutral countries of Luxembourg, Belgium and, in his original plan, much of the Netherlands. The bulk of the German Army would advance around the French fortifications, swing south-west of Paris before wheeling east, trapping the French Army against the German border in one gigantic encirclement. If everything went according to plan, France would be defeated in little more than six weeks, allowing Germany to transfer hundreds of thousands of soldiers by rail to face the slowly mobilising Russians.
By the time war broke out in 1914, however, Schlieffen was dead. His successor, Helmuth von Moltke the Younger, believed the original plan was too ambitious. He modified it by reducing the strength of the powerful right wing that was meant to sweep through Belgium and abandoned the invasion of the Netherlands altogether, hoping to preserve Dutch neutrality and keep valuable trade routes open. Historians still debate whether these changes fatally weakened the plan or whether Schlieffen’s original design had always been unrealistic.
On 2 August German forces occupied Luxembourg before crossing into neutral Belgium the following day. Belgium had refused German demands to allow its army free passage, and Britain – one of the guarantors of Belgian neutrality under the Treaty of London of 1839 – issued an ultimatum demanding Germany withdraw. When Germany ignored it, Britain declared war on 4 August 1914.
The invasion did not unfold as German commanders had hoped. The heavily fortified city of Liège offered unexpectedly fierce resistance, delaying the German advance for almost two weeks. Although enormous siege guns eventually reduced the Belgian forts, the delay disrupted Germany’s carefully planned timetable and allowed both France and Britain additional time to mobilise their forces. As German troops advanced through Belgium, reports emerged of civilians being executed and entire towns, most famously Leuven, being burned in retaliation for suspected resistance. Whether every allegation was accurate or not, these events became known collectively as the Rape of Belgium and were widely used in Allied propaganda, severely damaging Germany’s international reputation.
Meanwhile, the French launched Plan XVII, an ambitious offensive into Alsace and Lorraine in the belief that aggressive attacks would overwhelm the Germans. Instead, French armies were met by devastating machine-gun and artillery fire during the Battle of the Frontiers. Within weeks, the offensive collapsed with enormous casualties, forcing the French into a general retreat towards Paris.
To the north, the small but highly professional British Expeditionary Force (BEF) entered the war alongside France. On 23 August it fought its first major engagement against the Germans at the Battle of Mons. Although heavily outnumbered, British rifle fire inflicted surprising casualties and briefly slowed the German advance. Nevertheless, the BEF was eventually forced to retreat alongside the French, beginning what became known as the Great Retreat. By early September, German armies had advanced to within forty miles of Paris, and it appeared that the Schlieffen Plan was on the verge of delivering the rapid victory Germany had expected.
Instead, the offensive reached its breaking point. Exhausted soldiers had marched hundreds of miles in little over a month, supply lines stretched ever longer, and communication between German armies became increasingly difficult. Crucially, General Alexander von Kluck, commanding Germany’s powerful right wing, altered his line of advance by turning east of Paris rather than sweeping around the city as Schlieffen had envisaged. This exposed a dangerous gap between the German First and Second Armies.
Recognising the opportunity, the French commander Joseph Joffre ordered a massive counterattack. Reinforced by troops famously transported to the front in hundreds of Parisian taxis, French and British forces struck the exposed German flank during the First Battle of the Marne in early September 1914. Facing encirclement themselves, the Germans withdrew to the River Aisne. The Schlieffen Plan had failed.
Neither side was willing to accept defeat. Instead, both armies repeatedly attempted to outflank one another in a series of manoeuvres known as the Race to the Sea, extending the front further and further north until it reached the Belgian coast. The final major engagement of this race took place at the First Battle of Ypres, where repeated German attacks failed to break the Allied line.
Unable to outmanoeuvre one another, both sides began digging defensive trenches. Within weeks, a continuous line of earthworks, barbed wire and fortified positions stretched from the English Channel to the Swiss frontier. What many had expected to be a short, decisive war had instead become a brutal war of attrition that would last more than four years and claim millions of lives.