Nationalism started in Europe from the French Revolution and I am wondering what a good thesis topic would be.Like could I write about how it...

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.