Intense arms races and the glorification of military power.

The underlying causes are often remembered by the acronym :

The assassination of Archduke Franz Ferdinand of Austria-Hungary in Sarajevo on June 28, 1914. World Global Review: WWI in Ten Minutes

Strong devotion to national interests, particularly in the Balkans.

Competition for colonies and resources in Africa and Asia.

Complex secret treaties (Triple Entente vs. Triple Alliance) that turned a local conflict into a global one.