Final answer:
Nationalism is considered the most pivotal cause of World War I, as it fueled aggressive national policies and was integral in the support for war among the masses, particularly in the Balkans with the assassination of Archduke Franz Ferdinand, igniting the conflict.
Step-by-step explanation:
Most Responsible Cause for World War I
Among the four M.A.I.N. causes of World War I, arguably the most pivotal was Nationalism. Nationalistic fervor not only fueled aggressive policies and increased tension among European powers but also provided the driving force behind the various nations' public support for war. It was intensified by ethno-linguistic groups seeking independence and power within and outside of established states, leading to internal and external discord.
Nations were eager to demonstrate their dominance and defend their national honor, often at the expense of international relations. This was most tragically exemplified in the Balkans, where Serbian nationalists' desire for a Slavic state led to the assassination of Archduke Franz Ferdinand of Austria-Hungary, an event that triggered the war. The assassination pulled the various allied nations into conflict due to the extensive system of alliances that had been established.
Imperial ambitions and the competition for colonial territories further exacerbated nationalistic tensions, creating an environment ripe for conflict.