What was the reason for the outbreak of the First World War?

History clearly distinguishes between the reason and the reasons due to which this or that event happened.

The reason for the outbreak of the First World War was the assassination in the city of Sarajevo of the Archduke of the Austro-Hungarian Empire Franz Ferdinand on June 28, 1914.

The murder was committed by a Serbian high school student Gavrilo Princip, who is a member of a terrorist organization. Austria began preparations for war and delivered an ultimatum to Serbia. In response, Russia announced mobilization. Germany and Auto-Hungary used the mobilization in Russia as a pretext for the outbreak of hostilities.