Why did World War I start?
World War I started because decades of imperial rivalry, nationalist tensions, militarism, and entangling alliances had created a Europe primed for conflict. The assassination of Archduke Franz Ferdinand on June 28, 1914, set off a chain reaction of mobilizations and treaty obligations that escalated a Balkan crisis into a continental and then global war within six weeks.
The outbreak of World War I in the summer of 1914 remains one of the most studied and debated events in history. How did a political assassination in Sarajevo lead to a war that killed 20 million people? The answer lies in the interaction between deep structural causes and the specific decisions made during the July Crisis.
The deep causes were systemic. The European alliance system, designed to maintain balance and deter aggression, had instead created two armed camps that guaranteed any bilateral conflict would become multilateral. Germany and Austria-Hungary were bound by alliance; France and Russia were bound by theirs; Britain was informally committed to France. Arms races — particularly the Anglo-German naval rivalry — had fueled mutual suspicion and militarized public opinion. Imperialism had created flashpoints across the globe where great powers competed for territory and influence. Nationalism — particularly in the Balkans, where Serbs, Croats, Bosniaks, and others chafed under Austrian and Ottoman rule — provided the ideological fuel.
The immediate trigger was the assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, by Gavrilo Princip, a Bosnian Serb nationalist, on June 28, 1914, in Sarajevo. Austria-Hungary, seeking to crush Serbian nationalism, issued an ultimatum so extreme it was designed to be rejected. Serbia accepted most terms but not all. Austria-Hungary declared war on July 28.
From there, the alliance system and mobilization timetables took over. Russia, as protector of Slavic peoples, began mobilizing against Austria-Hungary. Germany, anticipating a two-front war, activated the Schlieffen Plan, which required attacking France through Belgium before Russia could fully mobilize. Germany's invasion of neutral Belgium brought Britain into the war on August 4. Within six weeks, all the major European powers were at war.
The critical insight is that no single cause was sufficient. Without the alliance system, the assassination would have remained a regional crisis. Without nationalism, the assassination might never have happened. Without militarism and arms races, governments might have found diplomatic solutions. Without imperial competition, the underlying tensions would have been lower. It was the interaction of all these factors — combined with specific failures of leadership and diplomacy during the July Crisis — that produced the catastrophe.