Final answer:
World War I was primarily caused by factors such as Militarism, Alliances, Imperialism, and Nationalism (MAIN), with the assassination of Archduke Franz Ferdinand serving as the immediate trigger.
Step-by-step explanation:
World War I was primarily caused by a combination of factors and events. The main long-term causes were Militarism, Alliances, Imperialism, and Nationalism (MAIN). These factors created an atmosphere of tension and competition among the European powers. Additionally, the assassination of Archduke Franz Ferdinand of Austria-Hungary served as the immediate trigger for the war.