How did World War I actually start?

The spark was a single assassination: in June 1914, a Bosnian Serb nationalist shot Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, and his wife in Sarajevo. But an assassination alone does not explain a war that killed millions; it lit a fuse that had been carefully laid over decades. Austria-Hungary, backed by Germany, issued harsh demands to Serbia and then declared war. Serbia's ally Russia mobilized, Germany declared war on Russia and its ally France, and when Germany invaded neutral Belgium to attack France, Britain entered too. Within about six weeks, a Balkan quarrel had pulled in all the major powers of Europe, each honoring alliance commitments like a row of falling dominoes.

The deeper causes are why historians say the war was waiting to happen. Europe had divided into two armed alliance blocs whose treaties turned any local conflict into a general one. Decades of militarism and an arms race, especially the naval rivalry between Britain and Germany, had built huge armies with rigid mobilization plans that, once started, were nearly impossible to stop. Aggressive nationalism, imperial competition over colonies, and instability in the Balkans, the powder keg of Europe, added fuel. The standard shorthand for the underlying causes is militarism, alliances, imperialism, and nationalism. So the honest answer is that the assassination was the trigger, not the cause: Europe had constructed a machine primed to convert one murder into a continental catastrophe, and in 1914 it did exactly that.