Answer:
World War I became an inevitable conflict due to a series of reasons, which include the increasing nationalist and militarist tendencies in Europe, imperialist expansionism and rigid alliances.
Nationalism and militarism peaked during the last years of the XIX century and the beginning of the XX century. Nationalism dictated that every nation (defined as groups of people that shared history, language, costums even ethnicity) should have its own State. This was clearly a disruptive force in a scenario were there were multinational states such as the Austro-Hungarian Empire or territorial disputes between countries such as the conflict between France and the German Empire over Alsace-Lorraine. Militarism also contributed to generate tensions between the different european actors. The arms races (both naval and on land) were characteristic of the years prior to the war.
On the other hand, the imperialist expansion of Europe in the late XIX century would create animosities between European countries, as the German Empire decided to embark in a "Welt Politik". This resulted in incidents, such as Fachoda or the Moroccan crises. While the distribution of territory among the imperial centers was not the decisive factor in generating the War, it contributed to tension the relations of the two forming alliances.
Finally, the rigid nature of the blocs formed in the early years of the XX century also was a key factor. Both, The Triple Entente and The Central Powers became the two actors of the European scenario in the pre WWI years. These blocs' actions resulted in immediate movilizations after the crisis generated by the assassination of Franz Ferdinand.