On the morning of 28 June 1914, a car carrying Archduke Franz Ferdinand, heir to the throne of Austria-Hungary, took a wrong turn in Sarajevo and stalled a few steps from a nineteen year old named Gavrilo Princip. He fired twice, killing the archduke and his wife Sophie. Within six weeks most of Europe's great powers were at war. The First World War, fought from 1914 to 1918, pitted the Allies (chiefly France, Britain, Russia, and later Italy and the United States) against the Central Powers (Germany, Austria-Hungary, the Ottoman Empire and Bulgaria). How a single murder became a world war is still one of history's great arguments.

A continent of rival blocs

By 1914 Europe was divided into two armed camps. The Triple Alliance of 1882 bound Germany, Austria-Hungary and Italy, though Italy stayed neutral when war came and joined the other side in 1915. Facing it stood France and Russia, allied since 1894, with Britain loosely attached through agreements in 1904 and 1907; together they were called the Triple Entente. Germany's leaders feared encirclement, and a naval arms race with Britain soured relations.

The Balkans were the most dangerous corner. As Ottoman power in southeastern Europe shrank, Austria-Hungary and Russia competed for influence there. Austria-Hungary annexed Bosnia and Herzegovina in 1908, angering Serbia, which hoped to unite the South Slavs under its leadership. Victories in the Balkan Wars of 1912 and 1913 nearly doubled Serbia's size and alarmed Vienna.

Thirty seven days

Princip belonged to Young Bosnia, a movement of mostly Bosnian Serb students, and the assassins were armed with help from the Black Hand, a secret society of Serbian army officers. The Serbian government had not ordered the killing, but Vienna saw a chance to crush Serbia. First it asked Berlin for support. On 5 and 6 July 1914, Germany promised to back Austria-Hungary whatever it decided, a pledge historians call the "blank cheque".

On 23 July Austria-Hungary sent Serbia an ultimatum with a 48 hour deadline. Serbia accepted most of the demands but refused to let Austrian officials take part in the investigation inside Serbia. Vienna declared war on 28 July. Russia, acting as Serbia's protector, ordered general mobilization on 30 July. Germany's war plan, drawn up years earlier under Count Alfred von Schlieffen, assumed a two front war and called for knocking France out quickly before Russia was ready. So Germany declared war on Russia on 1 August and on France on 3 August, and its armies entered neutral Belgium. Britain declared war on 4 August, after Germany invaded neutral Belgium, whose neutrality Britain had guaranteed by treaty in 1839. The German chancellor, Theobald von Bethmann Hollweg, reportedly complained that Britain was going to war over "a scrap of paper."

Who was to blame?

Historians have argued about this for a century. The peace treaty of 1919 placed responsibility on Germany and its allies, and German scholars spent the 1920s rejecting the charge. Many British writers between the wars, including the wartime prime minister David Lloyd George, came to describe the war as something no one fully intended: a slide over the edge by leaders who lost control of events.

In 1961 the German historian Fritz Fischer reopened the case. Using government archives, Fischer argued that Germany's leaders deliberately risked a general war to make Germany a world power, pointing to sweeping plans for annexations drawn up in September 1914. His defenders stress how readily Berlin pushed Vienna forward in July.

Critics replied that war aims written after fighting began do not prove intentions before it. Others widened the lens. The British historian A. J. P. Taylor stressed rigid railway mobilization timetables, which made it hard to stop once armies began to move. Sean McMeekin emphasized Russia's early, secret preparations for war. And in 2012 Christopher Clark, in "The Sleepwalkers," argued that responsibility was shared among several governments, each pursuing its own interests, misreading the others and failing to see the catastrophe ahead. Clark's critics answer that spreading blame thinly can underplay the special role of Berlin and Vienna, which chose escalation first.

Most historians today would agree that Austria-Hungary wanted a local war, that Germany encouraged it while accepting the risk of a wider one, and that Russia and France would not see Serbia crushed. Alliances, militarism, imperial rivalry and nationalism made a great war more likely, but none of them made this war inevitable. Choices made by a small number of men in a few weeks decided it.

Worked example: the blank cheque

Imagine you are Count Leopold Berchtold, Austria-Hungary's foreign minister, in early July 1914. You want to punish Serbia, but Russia might step in. Without German support you would probably settle for a diplomatic protest. With it, you can risk war. When Berlin replies that it will stand behind you even if Russia joins in, your calculation changes: a harsh ultimatum now seems safe, because you assume Russia will back down rather than face Germany too.

Now look from Berlin. Kaiser Wilhelm II and Bethmann Hollweg hoped for a quick local war, but they knew Russia might not back down. Fischer reads the pledge as a gamble for dominance; Clark reads it as one of many reckless moves made under pressure. Both draw on the same documents: the facts limit the argument, but interpretation decides how much weight each fact carries. When you meet any claim about who started a war, ask what evidence it relies on and what it leaves out.