The correct answer to this open question is the following.
It is true that Nationalism is when a person supports their own countries interests, especially when it comes to competing against another country. I think Nationalism contributed to the onset of World War I in that this sentiment create so much hate against other countries and confronted them.
Nationalism fomented a deep love and passion for a country to the extreme of patriotism. After the unification of Germany in 1871, nationalism invaded the sentiment of the Germans. However, nationalism was taken to the extreme with the outbreak of war in the Balkans region by the Slavic inhabitants.
Indeed, this issue was part of the assassination of the archduke of Austria, Franz Ferdinand and his wife. One of the major causes of World War I.