Answer:
The assassination of Austrian Archduke Francis Ferdinand.
Explanation:
The first World War began in 1914 because of the assassination of Austrian Archduke Francis Ferdinand. The shooting of the couple by a Serbian in Sarajevo led the Austro- Hungarian Empire pass threaten for an invasion in Serbia. All this started the European nations to form groups with alliances. The war extended and created two sides, the Allies and the Central Powers.