Why World War I Started

One assassination did not cause a world war; it lit a fuse on a bomb that had been assembled over decades. Historians often use the acronym MAIN: Militarism, Alliances, Imperialism, Nationalism. Europe's great powers had built huge armies (militarism) and locked themselves into rival defence pacts (alliances), so a fight between two countries would automatically drag in their partners. They competed fiercely for colonies and status (imperialism), and rising national pride made compromise look like weakness (nationalism). So when a Serbian nationalist killed Austria's heir in 1914, Austria attacked Serbia, Russia backed Serbia, Germany backed Austria, France and Britain were pulled in by their alliances. The tangle of promises turned a regional quarrel into a continental war within weeks.