How did World War 1 start?

A Serbian called Gavrilo Princip killed the Austro-Hungarian Duke Franz Ferdinand, in response Austro-Hungary threatened Serbia to give Princip up or else, but the Serbians didn't know who had killed Franz so could do nothing, Russia threatened Austro-Hungary not to invade, in turn the German Empire (who had a treaty with Austro-Hungary) threatened Russia, Britain and France threatened Germany as Britain and France had a treaty with Russia, Japan also helped Britain under the 1905 Anglo-Japanese Treaty. That was how it became a World War.