On 28 June 1914, Archduke Franz Ferdinand — heir to the Austro-Hungarian throne — was shot dead in Sarajevo by Gavrilo Princip, a Bosnian-Serb nationalist. Within six weeks, the major powers of Europe were at war. By November 1918, roughly 20 million people were dead. How does one assassination produce one of history's deadliest wars?
The assassination was the trigger, but it ignited a powder keg that had been building for decades. Historians sometimes use the acronym MAIN to summarise the underlying causes.
Europe in 1914 was like a room full of open barrels of gunpowder. The assassination in Sarajevo was the spark. Ordinarily, a political crisis between Austria-Hungary and Serbia would be a local affair — a small fire quickly extinguished. But the alliance system meant that if Austria-Hungary moved against Serbia, Russia would mobilise (to protect Slavic Serbia), which meant Germany would mobilise (as Austria-Hungary's ally), which meant France would mobilise (as Russia's ally), which meant Britain would consider mobilising (as France's informal ally and protector of Belgium). One spark, an interconnected room, and a chain reaction became almost automatic.
Militarism
The major European powers had been massively expanding their armies and navies for decades. Germany and Britain were in a naval arms race. Military planning had become incredibly detailed and committed — the German "Schlieffen Plan" required immediate mobilisation and invasion of France via Belgium the moment war seemed likely, with no intermediate options. Once mobilisation began, it was very hard to stop.
Alliance systems
Europe was divided into two armed camps: the Triple Entente (Britain, France, Russia) and the Triple Alliance (Germany, Austria-Hungary, Italy). These alliances meant a conflict between any two countries could automatically pull in four or six. What began as a bilateral dispute became a continental war within weeks.
Imperialism and nationalism
European empires were competing for territory and influence across the globe, creating repeated crises (particularly in Africa). Meanwhile, nationalist movements — peoples who identified by ethnicity seeking their own states — were destabilising the multi-ethnic Austro-Hungarian and Ottoman Empires. Gavrilo Princip was motivated by South Slav nationalism — the desire for a unified Serbian-Slavic state free from Austrian control.