The assassination of Archduke Franz Ferdinand in Sarajevo on June 28, 1914, lit the fuse, but World War I had been smoldering for decades. Historians often sum up the main causes with the acronym M.A.I.N.:

Militarism
European powers built massive armies and navies. Germany and Britain raced to outdo each other at sea, while France and Russia expanded on land. Military leaders gained influence over governments and pushed for war as a solution.

Alliances
Europe was divided into two rival blocs: the Triple Alliance (Germany, Austria-Hungary, Italy) and the Triple Entente (France, Russia, Britain). These agreements meant that a conflict between two nations could quickly pull in many more.

Imperialism
Competition for colonies in Africa and Asia created deep mistrust. Germany, a newer empire, resented the dominance of Britain and France.

Nationalism
Intense pride in national identity often turned into hostility toward neighbors. In the Balkans, Serbian nationalists wanted independence from Austria-Hungary, setting the stage for Gavrilo Princip’s deadly shot in Sarajevo.

The Spark

After the assassination on June 28, 1914, Austria-Hungary declared war on Serbia. Russia mobilized. Germany declared war on Russia and France. Germany invaded Belgium. Britain entered the conflict. A regional crisis turned into a global war.