Answer :

The direct cause of WW1 was the assassination of Franz Ferdinand by Serbian members of  secret military society, who had as their goal liberation of southern Slavs from the Austro-Hungarian rule.  Austria then set an ultimatum on Serbia in order to search for the guilty, but Serbia was backed by Russia....

There are of course many other reasons - this was just the direct one.
Gavrilo Princip shot the archduke and his wife sophia in 
Sarajevo 

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