What was the single most important cause of WWI?

It is obvious that there are many causes that lead to the start of the World War I. The first and most important in my eyes is the assassination of Archduke Ferdinand which upset Austria in many ways. There was a lot of assumption about why or who was behind the assignation but lead to the attack on Serbia. The second in my view is the invasion controlled by Germany into Belgium. Later France, Russia, Britain, Italy, Serbia and Hungary all started to mobilize their troops. If the assassination did not occur World War I may have not even started.