Question

What role did nationalism play in the outbreak of World War I?

Answer 1

Answer;
Nationalism convinced the people of Europe that their own nation could take on any military threat. This was a role nationalism played in the outbreak of world War I.

Explanation;
Nationalism reached a fevered pitch in Europe prior to the first World War. As a political tool, it was the belief that European technological, cultural, economic and military superiority was the cause for the subjugation of more backward economies and cultures. 
The pan-Slavic nationalism inspired the assassination of Archduke Franz Ferdinand in Sarajevo in June 1914, an event that led directly to the outbreak of World War I. 

Answer 2

Nationalism caused World War I. People put Nationalism in their country above other countries which caused a lot of conflict.

Is that enough information?