Which factor contributed most to the beginning of world war i?

Archduke Franz Ferninand of Austria was assassinated by a Bosnian Serb nationalist on their visit to the Bosnian capital of Sarajevo. The Serbian government was blamed for the attack which sparked conflict. Germany then gave Austria a "Blank Check" saying they would supply anything Austria needed for the war. Austria then went to war With Serbia causing allies of both Serbia and Austria to be dragged into the war causing World War I. After the war everyone blamed it on Germany. So, I guess it was the assassination of Archduke Franz Ferninand that contributed most.