Nationalism surely did help to start WWI. However, I am not sure that that is enough of a thesis topic. I think that I would (depending on how long the paper has to be) try to talk about the overall effects of nationalism in Europe.
For example, you could argue that nationalism had good effects. You could argue that nationalism led to the creation of countries like Italy and Germany. Nationalism helped to bring together all the little states that had made up Italy and Germany and make them into larger countries. You could argue that this is good because it reduced the amount of conflict between all the little states.
But nationalism did have bad effects as well. Nationalism helped lead to WWI because it encouraged people to think that their own country and their own kind of people were good and others were bad. This even helped to cause WWII and the Holocaust.
So I think I would try to write about all the impacts of nationalism and then evaluate whether nationalism was good for Europe.