What Started World War One. And why?

Austria-Hungary was looking for an excuse to wage a "preventative war" against Serbia so they could weaken them to take back territory in the Balkans, which had been claimed during the Balkan Wars. Their excuse was the assassination of Archduke Franz Ferdinand and his wife Sophie. A small war over a land dispute eventually became WW1 because both opposing countries were joined by the super powers all over the globe because of existing treaties from 75 years before this war.