Answer:
The assassination of Franz Ferdinand, archduke of Austria.
Step-by-step explanation:
When he assumed the throne, Franz Ferdinand wanted to transform the Austrian and Hungarian - at that time a double monarchy - into a third empire, recognizing the Slavic populations as part of the Empire. This idea was forcibly contrary to the Serbian interests whose desire was to create an independent state called Greater Serbia. To avoid this, the military organization know as Black Hand plotted the murder of the archduke, leading to a war.