Answer:
The assassination of the Austrian imperial heir Archduke Franz Ferdinand by a Serbian nationalist in Sarajevo in 1914 led to a declaration of war on Serbia.
Because Serbia was a Slavic Russian ally, the czarist government declared war on Austria-Hungary. Germany declared war on Russia because it had an alliance with Austria. Then France had to declare war on Germany and Austria-Hungary because of its committments to Moscow. And so on.
The policy of military alliances and secret diplomatic arrangements practiced in the decades before WWI, conducted recklessly to a large extent by all great powers of Europe, constructed a web of mutual compromises that led to the great continental and global conflict we know as WWI, causing a sort of chain reaction. The war went on for 4 years and caused unimaginable loss of lives and destruction.
Step-by-step explanation: