World War I was caused by a combination of long-term tensions and short-term triggers in early 20th century Europe. Historians typically identify four main causes: militarism, alliances, imperialism, and nationalism, often remembered by the acronym MAIN.
Steps
Militarism meant European powers built up massive armies and navies, creating an arms race where countries competed to have the strongest military forces and felt pressure to use them.
Alliances divided Europe into two opposing camps: the Triple Entente (France, Russia, Britain) and the Triple Alliance (Germany, Austria-Hungary, Italy), meaning a conflict between two countries could quickly pull in all their allies.
Imperialism created competition as European powers fought over colonies in Africa and Asia, leading to disputes and resentment between nations seeking to expand their empires.
Nationalism fueled intense pride and rivalry, particularly in the Balkans where Slavic groups wanted independence from Austria-Hungary, and countries like France desired revenge against Germany for earlier defeats.
The immediate trigger was the assassination of Archduke Franz Ferdinand of Austria-Hungary by a Serbian nationalist in June 1914, which set off the alliance system like dominoes and led to war declarations across Europe within weeks.
Worked example
When Austria-Hungary declared war on Serbia after the assassination, Russia mobilized to defend Serbia (due to Slavic nationalism and alliance). Germany then declared war on Russia (due to alliance with Austria-Hungary), which triggered France to mobilize (due to alliance with Russia), and Germany's invasion of Belgium brought Britain into the war (due to treaty obligations). This chain reaction shows how alliances turned a regional conflict into a world war.
Remember
World War I resulted from deep-seated tensions in militarism, alliances, imperialism, and nationalism that turned a single assassination into a global conflict involving multiple nations.