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Setting the scene
By 1914, Europe was divided into two armed camps: the Triple Entente (France, Russia, Britain) and Triple Alliance (Germany, Austria-Hungary, Italy). Nationalism, imperial rivalries, and a naval arms race between Britain and Germany created tension. The Balkans were a powder keg after Austria-Hungary annexed Bosnia in 1908, angering Serbia, which sought to unite South Slavs. All major powers had mobilization plans requiring rapid troop movements by rail, making diplomacy time-sensitive once mobilization began.
What happened
On June 28, 1914, Archduke Franz Ferdinand, heir to Austria-Hungary, was assassinated in Sarajevo by Gavrilo Princip, a 19-year-old Bosnian Serb nationalist. Austria-Hungary issued an ultimatum to Serbia on July 23, designed to be rejected. Serbia accepted most terms, but Austria declared war on July 28. Russia mobilized to defend Serbia, prompting Germany to declare war on Russia August 1 and on France August 3. Germany invaded Belgium to outflank France, bringing Britain in on August 4 due to the 1839 Treaty of London guaranteeing Belgian neutrality.
Why it still matters
World War I lasted four years, killed 20 million people, and destroyed four empires—German, Austro-Harian, Russian, and Ottoman. It introduced trench warfare, poison gas, tanks, and aerial bombing. The war's unresolved issues, especially the punitive Treaty of Versailles, directly caused World War II. It ended the European dominance of the world, spurred the Russian Revolution, and redrew the Middle East's map. The assassination in Sarajevo, by a teenager with a pistol, triggered a cascade of alliances and mobilizations that no leader could stop once started.
Background
The conflict introduced industrial-scale trench warfare in Europe.