On 28 June 1914, a Bosnian Serb nationalist assassinated Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, in Sarajevo. On its own, a single political assassination — even of a senior royal figure — had never been guaranteed to trigger a continent-wide war; Europe had absorbed comparable shocks before without global conflict following. This time, a network of alliances that had been built specifically to deter aggression instead turned a regional crisis into a war involving six major empires within roughly a month.
An alliance system built for deterrence, working in reverse
By 1914, Europe's major powers were bound together by a web of formal and informal alliances: Austria-Hungary and Germany on one side, France and Russia allied on the other, with Britain informally aligned with France and Russia as well. These alliances had been formed, in significant part, precisely to deter any single power from starting a war, on the theory that no country would risk aggression knowing it would face a combined alliance in response. When Austria-Hungary, backed by Germany's assurance of support, issued an ultimatum to Serbia after the assassination, that same alliance structure worked in exactly the opposite direction from its intended purpose — instead of deterring escalation, it pulled each allied power into the conflict in sequence as the crisis widened, each nation honouring commitments made in peacetime that were never actually tested under this kind of pressure.
From a Balkan crisis to a continental war in weeks
Austria-Hungary declared war on Serbia on 28 July, a month after the assassination. Russia, allied with Serbia, began mobilising its army in response. Germany, allied with Austria-Hungary and alarmed by Russian mobilisation, declared war on Russia, then on France (Russia's ally) days later, and invaded neutral Belgium en route to France — which brought Britain into the war as well, honouring its own commitments to Belgian neutrality and its informal alignment with France. Within roughly a week of Germany's declarations, most of Europe's major powers were formally at war, a remarkably fast escalation from a single regional assassination to a continental conflict, driven substantially by mobilisation timetables, alliance commitments, and military planning that had been designed for exactly this kind of rapid, sequential activation.
What we're still unsure about
Historians broadly agree on the sequence of diplomatic and military events leading from the assassination to general war, but there's genuine, long-running scholarly disagreement over how much weight to assign different underlying causes — the alliance system itself, aggressive German war planning (particularly the Schlieffen Plan's rigid mobilisation timetable), imperial rivalry, domestic political pressures within each power, and simple diplomatic miscalculation have all been argued as the primary driver by different historians at different times. This "war guilt" and causation debate has shifted considerably across the century since the war, and remains one of the more actively revisited questions in modern historical scholarship, without a single settled consensus on which factor mattered most.
This sits inside World War I: Causes, Course & Consequences, one of eight topics in Modern History, one of seven domains in History, one of seventeen subjects the app can quiz you on.