Russia, which viewed Serbia as a client state and could not afford to be seen abandoning it, ordered general mobilisation on 30 July. For Germany, Russian mobilisation meant the clock had started: the German military's entire strategic plan, known as the Schlieffen Plan, depended on fighting France and Russia sequentially rather than simultaneously, and that required moving fast. Germany declared war on Russia on 1 August and on France two days later. On 4 August, German troops invaded Belgium, which was guaranteed neutral under an 1839 treaty that Britain had signed. Britain sent an ultimatum demanding German withdrawal. When midnight came without a response, Britain and Germany were at war. In less than six weeks, a regional dispute in the Balkans had become a continental catastrophe.

The Schlieffen Plan called for eighty per cent of the German army to sweep through Belgium and northern France, encircle Paris, and knock France out of the war in roughly six weeks, before turning east to face Russia. The plan had been modified by the new German chief of staff, Helmuth von Moltke, who worried about his left flank and shifted more troops away from the decisive right wing. The opening advance was still dramatic: by the end of August 1914, the Allied left, which included the British Expeditionary Force, was in full retreat, and the French offensive into Alsace-Lorraine had failed with more than two hundred and sixty thousand casualties.

The plan's undoing came at the River Marne in early September. A gap opened between two German armies as they closed on Paris, and the French, reinforced by the British, counter-attacked into it, forcing the Germans back between forty and eighty kilometres. Both sides were too exhausted to press any further advantage. They dug in. Over the following weeks, in a series of flanking manoeuvres later called the Race to the Sea, each side tried to outflank the other and failed. By the end of 1914, an unbroken line of trenches stretched from the English Channel to the Swiss border, roughly seven hundred kilometres long. The war of movement was over. The war of attrition had begun.

The German Crown Prince, speaking to an American journalist shortly after the Battle of the Marne, was blunt: 'We have lost the war. It will go on for a long time but lost it is already.' He was wrong about the conclusion but right about the duration. The trenches that seemed temporary, to be abandoned once the next offensive succeeded, would define the war for four more years. The Germans, who usually chose their ground first, built their trenches to last, with deep dugouts and proper drainage. The Allied trenches were initially conceived as staging posts for the next attack, which never came as planned.

The Western Front was not the war's only theatre, but it was its centre of gravity. On the Eastern Front, Russia launched early attacks into East Prussia and Galicia, achieving their aim of drawing German forces away from France but suffering catastrophic defeats at Tannenberg and the Masurian Lakes in August and September 1914. By spring 1915, they had also retreated from Galicia. In the south, Austria's opening invasion of Serbia, which began on 12 August 1914, was repulsed with heavy losses in what has been called one of the major upset victories of the century. Austria was forced to keep substantial forces on the Serbian front, weakening its efforts against Russia.

The trenches produced a new kind of warfare for which no army was prepared. Pre-war tactics had been built around open movement and the individual rifleman. What the armies found instead was a landscape dominated by barbed wire, machine guns, and artillery, all of which favoured the defender. Mass infantry advances across open ground became exercises in organised killing. Both sides struggled for years to find a way through, and in the meantime the casualties mounted in numbers that had no precedent in the history of war.

On 22 April 1915, at the Second Battle of Ypres, the Germans introduced a new weapon: chlorine gas, released from cylinders along a four-kilometre front. It violated the Hague Conventions, which had banned asphyxiating gases in warfare. It was also, in the short term, terrifyingly effective. Both sides subsequently developed and deployed chemical weapons throughout the war, inflicting roughly one and a half million casualties in total, of which around ninety thousand were fatal. The gas mask became as standard an item of equipment as a rifle. Soldiers learned to live, and die, in a landscape that could kill them with the wind.

The Battle of Verdun, which the Germans launched in February 1916 and which ground on until December, was designed not to capture French territory but to bleed the French army to exhaustion. The result was mutual destruction on an almost incomprehensible scale: somewhere between seven hundred thousand and nearly a million casualties combined. Verdun became a word that carried its own weight in French memory, a symbol of endurance and sacrifice that the French nation has never entirely set down.

The Battle of the Somme, beginning on 1 July 1916, opened as a joint Anglo-French offensive intended to relieve pressure on Verdun. The opening day alone produced fifty-seven thousand five hundred British casualties, including nearly twenty thousand dead. It remains the single bloodiest day in the history of the British Army. By the time the offensive ended in November, it had cost roughly four hundred and twenty thousand British casualties, two hundred thousand French, and five hundred thousand German. The ground gained could be measured in kilometres. The dead could not be counted quickly enough to bury them all.

The technology of the war evolved constantly, though rarely fast enough to break the stalemate cleanly. Tanks appeared on the Somme in September 1916, lumbering and unreliable but capable of crossing trenches and suppressing machine-gun fire. Aircraft moved from reconnaissance to ground attack to strategic bombing. Artillery became increasingly sophisticated, using indirect fire directed by aircraft and field telephones rather than line of sight. None of these innovations ended the war quickly. They did change the nature of it, and by 1917 both sides were fighting a war that bore almost no resemblance to the one their generals had planned.

The war was never only a European affair. It reached into the Ottoman Empire, into Africa, into the Pacific, and ultimately across the Atlantic. The Ottoman Empire joined the Central Powers in November 1914 and immediately threatened both Russia's Caucasian territories and Britain's communications with India through the Suez Canal. The Ottomans also used the cover of the war for something far darker: the systematic extermination of their Armenian population. Beginning in 1915, mass deportations and executions killed an estimated one and a half million Armenians. The Ottomans carried out similar campaigns against Assyrian and Greek Christian communities. These events are recognised by historians as genocide, though the government of Turkey continues to deny it.

At Gallipoli in 1915, Britain and France attempted to knock the Ottomans out of the war by forcing the Dardanelles strait, landing troops on the peninsula, and advancing on Constantinople. The campaign failed entirely. Ottoman forces, with German support, repelled every assault. The soldiers from Australia and New Zealand who fought there, the ANZACs, suffered enormous casualties in a campaign that achieved nothing militarily. But Gallipoli became something else for those nations: the first major war they had fought as Australians and New Zealanders rather than simply as British subjects. Anzac Day, marked every April, commemorates that defining and devastating moment.

In Mesopotamia, the story was more complex. An early British defeat at the siege of Kut in 1915 and 1916 was reversed when reorganised British forces captured Baghdad in March 1917. The Arab Revolt, encouraged by the British Foreign Office and launched in June 1916, gradually drove Ottoman forces out of much of the Arabian peninsula, culminating eventually in the Ottoman surrender of Damascus. In the Sinai and Palestine, General Edmund Allenby led a campaign that captured Jerusalem in December 1917, after victories at Beersheba and Mughar Ridge. The Ottoman Empire was being dismantled from multiple directions simultaneously.

On the Eastern Front, a moment of genuine Allied brilliance came in June 1916 with the Brusilov offensive, in which Russian forces under General Alexei Brusilov achieved a breakthrough against the Austrians in Galicia that advanced hundreds of kilometres and took hundreds of thousands of prisoners. It was one of the most successful offensives of the entire war. But it fatally weakened both the Austrian and Russian armies, accelerated the disillusionment of Russian soldiers with the war, and contributed directly to the revolutions that would follow.

The naval war was fought primarily in the North Sea and the Atlantic. Britain imposed a naval blockade on Germany from the war's opening weeks, cutting off vital supplies and contributing to civilian food shortages that killed nearly half a million Germans over the course of the war. Germany responded with submarine warfare against Allied shipping. In 1915, a German submarine sank the passenger liner RMS Lusitania, killing nearly twelve hundred people including many Americans, and provoking a fierce diplomatic protest from Washington. Germany temporarily restrained its submarines, but the strategic logic of cutting Allied supply lines was irresistible.

By 1916, the war had drawn in nations from across Europe and beyond. Italy, which had been a member of the Triple Alliance before the war but declared neutrality in 1914, was persuaded to join the Allies by the secret Treaty of London in April 1915, in exchange for promises of Austrian territory. It declared war on Austria-Hungary on 23 May 1915. The Italian campaign was fought in some of the most brutal terrain imaginable, high in the Alps and Dolomites, where trench lines had to be carved through rock and ice. The Italian commander, Luigi Cadorna, launched eleven successive frontal assaults along the Isonzo River between 1915 and 1917, making minimal progress at enormous cost.

Bulgaria joined the Central Powers in October 1915, and its entry transformed the Balkan campaign. With a combined Central Powers force of six hundred thousand troops pressing from multiple directions, Serbia was overrun in little more than a month. The Serbian army, facing certain destruction, retreated across the mountains of Albania in winter. The survivors were eventually evacuated to Greece. Romania entered on the Allied side in August 1916, attracted by the promise of Transylvania and emboldened by the Brusilov offensive, but was quickly overwhelmed by a German-led counter-attack. Bucharest fell to the Central Powers on 6 December 1916.

India's contribution to the Allied war effort is a chapter that tends to be overlooked. Between 1914 and 1918, an estimated one and a half million Indian soldiers and labourers served in Europe, Africa, and the Middle East. About a hundred and forty thousand served on the Western Front. Nearly forty-eight thousand were killed. The war had been presented to Indian leaders as an opportunity: support Britain now, and Home Rule would follow. That promise was not kept. The suffering of the war, and the betrayal that followed it, bred a disillusionment that would fuel the campaign for full independence led by Mahatma Gandhi.

The French Army mutinies of spring 1917 brought the Western Front to a hidden crisis. After the disastrous Nivelle offensive in April, in which French forces suffered a hundred and thirty-five thousand casualties in the first two days of fighting against the heavily fortified Hindenburg Line, French soldiers began to refuse orders. By early May, collective indiscipline had spread to fifty-four divisions. More than twenty thousand men deserted. The French high command managed to suppress public knowledge of the mutinies for decades. For weeks, there was effectively no functioning French offensive capacity on the Western Front.

In April 1917, the United States entered the war. The immediate cause was Germany's decision, taken in February, to resume unrestricted submarine warfare against all shipping in the Atlantic, knowing it would bring America in but gambling it could strangle Allied supply lines before American troops arrived in force. The gamble failed. Congress declared war on 6 April. The American army in April 1917 numbered fewer than three hundred thousand men; by the end of November 1918, two million had crossed to France. The first significant American offensive was the Meuse-Argonne operation in late September 1918, which became one of the largest military operations in American history.

Russia did not survive the war it had helped to start. By the end of 1916, Russian casualties totalled nearly five million killed, wounded, or captured. Food shortages and high prices had destabilised the major cities. In March 1917, Tsar Nicholas the Second ordered troops to suppress strikes in Petrograd. The troops refused to fire. The State Duma forced Nicholas to abdicate. A Provisional Government was established, which declared its intention to continue the war. But a rival power centre, the Petrograd Soviet, refused to disband. The result was paralysis, and the paralysis opened the door for Vladimir Lenin.

Lenin had been in exile in Switzerland when the revolution broke out. The German government, recognising that a Bolshevik Russia would take the country out of the war, arranged his transport home in a sealed railway carriage. He arrived in Petrograd on 16 April 1917 and immediately demanded an end to the war. The Bolsheviks seized power in November. In December, they signed an armistice with Germany. On 3 March 1918, they signed the Treaty of Brest-Litovsk, ceding Finland, Estonia, Latvia, Lithuania, and large parts of Poland and Ukraine to the Central Powers. Russia's three years of sacrifice had bought it nothing but revolution and territorial dismemberment.

With Russia out of the war, Germany transferred large numbers of troops to the Western Front and launched its great spring offensive, Operation Michael, on 21 March 1918. German forces advanced sixty kilometres in the first days, an unprecedented gain on the Western Front. For a moment it seemed the stalemate might break in Germany's favour. But the advance outran its supply lines. The Germans lacked tanks and motorised artillery to consolidate what they had taken. The terrain they had crossed was shell-torn and impassable. A series of follow-up operations, including attacks toward the Channel ports and a thrust toward Paris, each achieved initial gains and then stalled. Germany had spent its last reserves of offensive power.

The Allied counter-offensive began on 8 August 1918, later called the Hundred Days Offensive. The opening assault at Amiens, using massed tanks and coordinated artillery, shattered four German divisions and advanced twelve kilometres on the first day alone. German General Erich Ludendorff called it the Black Day of the German Army. What followed was a sustained advance that did not stop. The Hindenburg Line, Germany's last major defensive system, was breached by British and Dominion forces at Cambrai on 8 October. In the final week of September alone, British forces took more than thirty thousand prisoners.

The collapse of Germany's allies came quickly. Bulgaria was the first to fall, signing the Armistice of Salonica on 29 September 1918, after Serbian and French forces broke through the Macedonian front at the Battle of Dobro Pole in a rare clean breakthrough, something almost unseen in four years of trench warfare. Kaiser Wilhelm the Second, informed of the Bulgarian surrender, sent a telegram to the Bulgarian tsar that captured the moment with bitter clarity: 'Disgraceful! Sixty-two thousand Serbs decided the war!' On the same day, the German Supreme Command told the Kaiser that Germany's military position was hopeless.

The Ottoman Empire signed the Armistice of Mudros on 30 October 1918. Austria-Hungary followed on 3 November with the Armistice of Villa Giusti, signed after the Battle of Vittorio Veneto had effectively destroyed the Austro-Hungarian army as a fighting force. During the last weeks of October, declarations of independence had been proclaimed in Budapest, Prague, and Zagreb. The Habsburg monarchy, which had ruled much of central Europe for centuries, simply dissolved. Four empires, the Russian, the German, the Austro-Hungarian, and the Ottoman, had been destroyed by a war that had begun, in part, over the ambitions of one of them in a small Balkan city.

Germany itself came apart from within. In late October, sailors at the naval ports of Wilhelmshaven and Kiel refused orders to put to sea for a final, suicidal battle against the Royal Navy. The mutiny spread. Workers' and soldiers' councils formed across German cities. A republic was proclaimed on 9 November 1918. On the same day, Kaiser Wilhelm the Second abdicated and fled to the Netherlands. Two days later, at five in the morning on 11 November 1918, German representatives signed the Armistice in a railway carriage in the forest of Compiègne in northern France. The guns fell silent at eleven o'clock that morning.

The numbers that followed were almost impossible to absorb. An estimated fifteen to twenty-two million people had been killed, military and civilian combined. Of the sixty million European soldiers mobilised, roughly eight million were dead, seven million were permanently disabled, and fifteen million had been seriously wounded. France had mobilised seven and a half million men; one and a half million were dead, three million more were injured. Germany lost more than fifteen per cent of its active male population. Austria-Hungary lost more than seventeen per cent. The war had also accelerated a global influenza pandemic, the Spanish flu, which killed at least seventeen million more people worldwide. Eight million horses, donkeys, and mules died alongside the men.

The human cost carried its own particular textures of horror. In France alone, around fifteen thousand men sustained severe facial injuries so disfiguring that they became objects of social avoidance; they were known as the gueules cassées, the broken faces. In Serbia, louse-borne typhus had killed two hundred thousand people in 1914 alone. In Lebanon, famine killed roughly a hundred thousand. The trenches produced diseases that soldiers had no name for yet: trench foot from standing in waterlogged dugouts, trench fever from lice, and the slow destruction of minds that would later be called shell shock.

The Paris Peace Conference opened in January 1919, and the settlement it produced would shape the next twenty years of world history. The Treaty of Versailles, signed on 28 June 1919, imposed sweeping terms on Germany. Under Article 231, the so-called War Guilt Clause, Germany was required to accept responsibility for all the losses and damage caused by the war, a statement designed to provide a legal basis for the reparations that followed. The total reparations bill was eventually set at a hundred and thirty-two billion gold marks, though Allied experts privately acknowledged Germany could not pay the full sum.

Germany lost significant territories: Alsace and Lorraine to France, large areas of the east to the newly reconstituted Poland, and all its overseas colonies. Its army was reduced to a hundred thousand men. The Rhineland was occupied. German historian Hagen Schulze described the result as leaving Germany under legal sanctions, deprived of military power, economically ruined, and politically humiliated. Most Germans regarded the treaty as a diktat, an imposed humiliation rather than a negotiated peace. The legend of the stab in the back, the claim that Germany had been betrayed from within rather than defeated in the field, took root almost immediately and would poison German politics for the next two decades.

The map of Europe was redrawn entirely. Four empires disappeared. In their place came new nations: Poland re-emerged as an independent state after a hundred and twenty-three years of partition. Czechoslovakia, Yugoslavia, Finland, and the Baltic states of Estonia, Latvia, and Lithuania were created or recognised. Austria-Hungary was partitioned along rough ethnic lines, though the boundaries satisfied almost no one. Hungary alone lost sixty-four per cent of its pre-war population through the new borders. The dissolution was not clean: it created new minorities, new resentments, and new border disputes that would fester through the 1920s and 1930s.

The League of Nations, conceived by American President Woodrow Wilson as the instrument that would prevent future wars, was established by the Treaty of Versailles. Wilson had gone to war partly to ensure the United States would shape the peace. He succeeded in getting the League created. But the US Senate refused to ratify the treaty, and America never joined the organisation Wilson had built. The League was left to manage European instability without the participation of the world's most powerful economy. It would fail at the task. Within twenty years, the conditions created by the peace of 1919 had produced a second, larger war.

The war transformed societies in ways that outlasted any treaty. Women had entered the industrial workforce in unprecedented numbers to replace men sent to the front, a shift that accelerated the long campaign for voting rights. Veterans who came home found they could only speak about what they had seen with others who had been there. They formed associations, built memorials, and tried to explain a war that resisted explanation. The poem written in 1915 by a Canadian army doctor named John McCrae, 'In Flanders Fields', became one of the most recited poems in the English language, read aloud every November at Remembrance Day services around the world.

Britain still owed the United States more than four billion dollars in war debt in 1934. The final instalment was paid in 2015. Germany made its last reparations payment in October 2010. The financial architecture of the war lasted nearly a century. The physical residue lasted longer still: as late as 2007, unexploded shells at Verdun and the Somme continued to surface from the earth, and in some places the vegetation had still not fully recovered from what had been done to the ground between 1914 and 1918.

The war's defining irony was that it was fought, at enormous cost, to resolve a crisis that the killing only deepened. The alliances built to prevent war had made it total. The victory won at such sacrifice produced a peace that planted the seeds of another conflict. The empires destroyed by the war left vacuums filled by nationalisms and resentments that the League of Nations could not contain. 'The war to end all wars' did not end wars. It ended the world that had made them possible, and left behind a different, angrier world with no clear rules for what came next. Twenty-one years after the Armistice, the guns began again.

What the First World War destroyed cannot be restored: the lives of an estimated fifteen to twenty-two million people, the stability of a continent, the certainty that progress and civilisation moved in one direction only. What it left behind was a question that every subsequent generation has had to answer: how does a world that has seen industrialised slaughter on that scale choose to organise itself? The answers given at Versailles were inadequate. The answers given after 1945 were better, though never complete. The question itself, of how to prevent the mechanisms of alliance, nationalism, and militarism from dragging civilised nations into catastrophe, remains as open now as it was on the morning of 11 November 1918, when the last shell was fired and the silence, for a moment, was absolute.