On a hot Sunday morning, 28 June 1914, an open-topped car carrying Archduke Franz Ferdinand took a wrong turn in Sarajevo. As the driver braked to reverse, a nineteen-year-old Bosnian Serb named Gavrilo Princip stepped forward and fired twice. Within the hour, the heir to the Austro-Hungarian throne and his wife Sophie were dead.

Political murders were not rare in this era. Kings, tsars and presidents had been killed before without bringing down the world. Yet five weeks after Sarajevo, the great powers of Europe were at war. The machinery that turned a street shooting into a world war had been built, piece by piece, across the long nineteenth century.

After Germany unified in 1871, its chancellor Otto von Bismarck feared being surrounded by enemies. His answer was a web of treaties. By 1882 Germany, Austria-Hungary and Italy had formed the Triple Alliance. The logic was simple: if everyone knew who stood behind whom, no one would dare start a fight.

Rival powers reached the same conclusion. France and Russia allied in 1894. Britain, long proud of its 'splendid isolation', settled its colonial quarrels with France in 1904 and with Russia in 1907. Together they formed the Triple Entente. Europe now stood in two armed camps, each convinced it was only defending itself.

The danger lay in how these commitments were connected. Austria-Hungary wanted to punish Serbia, which it blamed for the assassination. Russia saw itself as Serbia's protector. In early July, Germany promised Vienna unconditional support, the famous 'blank cheque'. France stood beside Russia. A Balkan quarrel now ran like a fuse to Berlin, Paris and St Petersburg. Even Italy's neutrality could not break the chain. Alliances meant to discourage war had made it almost impossible to keep a war small.

Takeaway

Systems built to make conflict unthinkable can, under pressure, make it unstoppable. Deterrence only works while each side believes the other will be the one to back down.

The railway had transformed war as completely as it had transformed trade. An army could now reach a frontier in days rather than weeks, but only if everything was planned in advance. General staffs spent years drawing up timetables for thousands of trains, with every carriage, horse and rifle scheduled almost to the minute.

Germany's plan, first sketched by Count Alfred von Schlieffen, assumed a war on two fronts. Russia, vast and slow to mobilise, would be held off while the German army swept through neutral Belgium to defeat France in about six weeks. Then the troops would be carried east by rail. By 1914 Germany had no separate plan for fighting Russia alone.

This made time itself the enemy. When Russia ordered general mobilisation on 30 July, German generals insisted they could not wait. On 1 August, Kaiser Wilhelm II asked whether the army could march east only. His chief of staff, Helmuth von Moltke, told him it was impossible: a year of careful planning could not be undone. Germany declared war on Russia that day, on France two days later, and on 4 August marched into Belgium, bringing Britain into the war.

Takeaway

When plans leave no room for delay, they quietly take decisions away from the people meant to make them. Speed, once an advantage, can become a trap.

Ever since 1914, people have argued about who was to blame. David Lloyd George, later Britain's wartime prime minister, wrote that the nations had 'slithered over the brink'. In 2012 the historian Christopher Clark called the leaders of 1914 sleepwalkers: watchful but unseeing, each acting sensibly by their own lights while walking together into catastrophe.

Others see deliberate choice. In the 1960s the German historian Fritz Fischer argued that Germany's leaders welcomed war as a chance to break out of encirclement and become a world power. Vienna was determined to crush Serbia. Russian and French leaders also preferred war to losing prestige. Few wanted a long war, but many decided a short one was a risk worth taking.

Both views reveal something about the world that ended that summer. The long nineteenth century had been an age of remarkable confidence: in progress, in industry, in the nation and in empire. Leaders expected war to be quick and decisive, like Prussia's victories in 1866 and 1870. They trusted that the machines and institutions they had built could be controlled. The events of 1914 showed how dangerous that faith could be.

Takeaway

Catastrophes rarely need pure villains or pure accidents. They often grow from confident people making reasonable-seeming choices inside systems they no longer fully understand.

On the evening of 3 August, Britain's foreign secretary Sir Edward Grey reportedly watched the lamps being lit in London and said: 'The lamps are going out all over Europe.' Within four years, four empires had collapsed: German, Austro-Hungarian, Russian and Ottoman.

Yet the lessons of 1914 still shape our world. Collective security, crisis hotlines and the habit of buying diplomats time were all built by people who remembered how quickly that summer slipped out of control.