What caused World War I?
World War I (1914–1918) was caused by a volatile combination of imperial rivalries, entangling alliance systems, militarism, and nationalist tensions in Europe. The assassination of Archduke Franz Ferdinand of Austria-Hungary in June 1914 triggered a chain reaction of treaty obligations that pulled the major European powers into a catastrophic global conflict within weeks.
World War I was not the result of a single cause but the eruption of pressures that had been building across Europe for decades. Understanding what caused the Great War requires examining the structural conditions that made a general European conflict possible and the specific events that triggered it.
The alliance system transformed a regional crisis into a continental war. By 1914, Europe was divided into two rigid blocs: the Triple Alliance (Germany, Austria-Hungary, Italy) and the Triple Entente (France, Russia, Britain). These alliances, originally designed to deter aggression, meant that a conflict between any two powers would automatically draw in the rest. When Austria-Hungary declared war on Serbia after the assassination of Archduke Franz Ferdinand in Sarajevo on June 28, 1914, Russia mobilized to defend Serbia, Germany mobilized to support Austria-Hungary, France mobilized to honor its alliance with Russia, and Germany's invasion of Belgium brought Britain into the war.
Imperialism and nationalism had created a powder keg of resentments. Germany, unified only in 1871, demanded its 'place in the sun' — colonies, naval power, and prestige to match Britain and France. The Balkans were a volatile mix of competing nationalist aspirations, with Austria-Hungary trying to suppress South Slavic nationalism while Serbia sought to unite all Slavs under its leadership. France nursed a grievance over the loss of Alsace-Lorraine to Germany in 1871.
Militarism made war seem not only possible but inevitable. Every major power had expanded its armies and navies, developed detailed mobilization plans, and cultivated a culture that glorified military strength. The arms race — particularly the Anglo-German naval rivalry — heightened suspicion and paranoia. Military leaders operated on timetables that left little room for diplomacy once mobilization began. Germany's Schlieffen Plan, which required a rapid strike through Belgium to knock out France before turning east against Russia, meant that any German mobilization was effectively a declaration of war on two fronts.
The result was a war that virtually no one had wanted in its actual form — a four-year bloodbath that killed approximately 20 million people and wounded another 21 million, toppled four empires, and set the stage for an even more devastating conflict just two decades later.