Final answer:
Nationalism was a long-term cause of the war that encouraged competition between nations and ethnic groups seeking independence.
Step-by-step explanation:
Nationalism is the long-term cause of the war that encouraged competitiveness between nations and various ethnic groups to attempt to create nations of their own. This sense of pride and devotion to one's nation created unity but also led to a thirst for power and glory. European powers wanted to showcase their strength and compete with one another, contributing to the concept of nationalism. Ethnic groups in smaller nations, like Serbia, also had strong nationalistic sentiments and desired independence to form their own identity.