Final answer:
The underlying causes of World War I were nationalism, opposition to foreign rule, and simmering rivalries between the Great Powers. The most influential cause was subjective, but nationalism played a significant role.
Step-by-step explanation:
The underlying causes of World War I were nationalism, opposition to foreign rule, and simmering rivalries between the Great Powers that were exacerbated by treaties requiring allies to enter a war once it began. These causes can be summarized using the acronym MAIN: Militarism, Alliances, Imperialism, and Nationalism.
The most influential cause of World War I is subjective and open to debate. However, one could argue that nationalism played a significant role in driving the global conflict. Nationalism fueled a sense of patriotism and loyalty to one's own country, which contributed to the escalation of tensions and the desire for dominance in Europe.