Category : Causes of WWI
There were mainly two kinds of nationalism in 20th century Europe before WWI. (World War I Nationalism, Funfront)
One of them was the desire of subjected people for their independence. For example the Slavic people in Bosnia and Herzegovina wanted to be no longer controlled by Austria-Hungary but be part of Serbia (The Top 5 Causes That Led to World War I, About.com Education). The desire of gaining independence caused Serbia to ally with Russia and Russia later helped Serbia to fight with Austria-Hungary. Since Austria-Hungary was friend of Germany, later Germany get involved in the war with Serbia and Russia. The involvement of more countries let to the instability to the current peace in Europe that eventually led to war.
The other kind of nationalism at that time was the desire of strong nations for dominance and prestige, (World War I Nationalism, Funfront) like Germany. After Germany was united after Franco-Prussian war it became one of the countries which had the strongest economic and military power in Europe. The desire to expand territories could never be stopped and this desire definitely reflected on more battles caused by Germany. Germany became more aggressive after it became stronger that it wanted to build up its influence in every part of the globe, because of that, Germany had lots more serious conflicts with other major powers in Europe (World War I Nationalism, Funfront).
These were the two main kinds of nationalism that were also a cause of WWI.