Nationalism was one of the many causes of World War I. Many countries began to develop an extreme sense that their nation, way of life, and culture were superior to those of any other nation. Thus, these countries wanted to conquer other lands to spread their way of life.
Countries like Germany had very few colonies outside of their borders. Germany wanted to gain control of colonies, and Germany most likely was going to have to fight other colonial powers in order to accomplish this. Germany began to build up its military and had no concern about using the military. Germany believed its military was better than the military of any other country, so if a war began Germany believed it would win that war. Germany believed its country and military were so good, it could do anything it wanted to do.
Nationalism played another role in the start of World War I. There were many people who were being ruled by people who weren’t from the same nationality. There were Serbians being ruled by Austria-Hungary. The Serbians wanted to be ruled by Serbians. When Austria-Hungary refused to give some land of Austria-Hungary where many Serbians lived to Serbia, a group of Serbian nationals carried out a plan to kill the next King of Austria-Hungary, Franz Ferdinand. This ultimately led to a series of events that helped start World War I.
Nationalism contributed to unrest because it led people in the Austro-Hungarian Empire to be unhappy with the fact that they were part of that empire. This empire was ruled by people who were ethnically German. But many of the people they ruled were not ethnically German. Among these were the Slavs who lived in the Balkans. They were very unhappy with the idea of being ruled by people outside their own ethnic group. This led to unrest among them and, eventually, to the assassination of the Austrian archduke that set off the war.