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					Originally Posted by ariel
					
				 
				Perhaps, in retrospect, it was the most consequential war: millions of dead for no good reason except for the German wish to become an European hegemon and the unwillingness of Britain and France to allow it, 4 Empires destroyed,     peculiarly  fratricidal one ( German, Russian and British rulers were cousins), and the ultimate results was the mutation of Germany into a Nazi state and of Russia into a Stalin's Gulag with the horrifying WWII  being an inevitable outcome occuring only less than 20 years after " the war to end all wars"....   
 
We are still living with the consequences of the disastrous decisions of European rulers in August 1914.... 
			
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 Not quite as simple a cause as described by ariel but rather than re state what Wiki says, here is the link 
https://en.wikipedia.org/wiki/World_War_I As you will read, there were several events which eventually lead to the wider conflict.
Stu