What was the main cause of WWI?

Britain and France both had empires that were making them rich, Germany wanted one too and was afraid that if it didn't do something it would soon be starved of resources such as coal and iron and that it would have to play second fiddle to Britain and France, so Germany decided that if it couldn't build an empire in Africa, Asia or India, it would build one in Europe. So it armed up and waited for an opportune moment to strike. It needed an excuse to declare war to make it seem legal, and when Arch duke Franz Ferdinand was inated it used this as an excuse to start a war.