What was the "spark" needed to ignite World War I?

The spark that set off World War I came on June 28, 1914, when a young Serbian patriot shot and killed Archduke Franz Ferdinand, the heir to the Austro-Hungarian Empire (Austria), in the city of Sarajevo. The assassin was a supporter of the Kingdom of Serbia, and within a month the Austrian army invaded Serbia.

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"germany's geographic location was an advantage when it was on the offensive in the war because it was situated next to an extremely wide river, the rhine, which was difficult for the allies to cross."