Final answer:
Nationalism caused World War I due to increased rivalry and competition for material resources and territory.
Step-by-step explanation:
Nationalism caused World War I because the rivalry between nations intensified, leading to competition for material resources and territory. Nationalism fostered a sense of pride and devotion to one's nation, but it also fueled desires for power and glory. As European powers sought opportunities to showcase their strength, they competed for colonies and sought to expand their territories. This competition for land and resources eventually led to the outbreak of World War I.